It sounds like you've really done your research and picked an excellent vet for the neuter!
Males remain fertile for a while post-neuter, but since females typically mature around 4-6 months old and 12 weeks is considered the cut-off for keeping an intact female with a fertile male (as a precaution against early bloomers), that won't be a concern here - he'll definitely be infertile by the time she's 12 weeks old.
Depending on how old she needs to be for a spay, she could be 7 months old before you can start truly bonding them (since it takes up to a month for females' hormones to dissipate after a spay), which could make for a long 5 1/2+ months of keeping them housed separately... but it sounds like you're totally on board with the idea of a bonded pair being a long ways away
.
You mentioned she'll be larger than Felix; Mini lops (aka Holland lops in the US) are quite tiny, with an adult weight of 2-3 lbs, assuming they inherit the dwarf gene... so does that mean she's a either a Dwarf lop (aka Mini lop in the US; adult weight of 4-5 lbs) or an even larger lop breed? Very experienced vets are often willing to spay as young as 4 months of age IF the rabbit meets a certain weight/size threshold - for example, my San Antonio vet spays at 4-6 months old so long as they're at least about 1 kg, aka 2.2 lbs - Nala and Gaz were spayed at 5 months old, though if I'd known he had a specific weight threshold, I probably could've gotten them done a couple weeks sooner (when they were in for an appointment at three months old, I asked if I could go ahead and book their spays for a month later and he said he'd prefer to wait an extra month because of their small size; they were around 1.4 kg by their spay date).
In other words, if she's a medium sized breed rather than a dwarf (or possibly if she's a Mini (aka Holland) who didn't inherit the dwarf gene), then there's a good chance that she could meet your vet's requirements for spaying *prior* to her hormones actually kicking in. If that ended up being the case, you could avoid the one month waiting period for hormones and start officially bonding them as soon as her post-spay restriction period (typically 7-10 days) ended.
I think letting them have supervised time together but keeping them separately when you're not around to break up a fight is a solid plan. If you have their enclosures set up so that they're able to see and smell each other yet unable to fight through the bars, that's an excellent way to preserve and reinforce their "baby bond" and give you a great head-start towards an adult bond. Assuming they hit it off and forge a tentative bond prior to her spay, you'll want to make arrangements to bring him with her (even if it's in a different carrier) when she goes in for the spay so that he can wait in the back for her to get out of surgery - otherwise, he could take offense to her coming home smelling like "weird vet's office." Essentially, you'll need to treat them as a bonded pair despite keeping them separately when unsupervised (ie if one goes somewhere, they both have to go).
~~~~~
As a note, I'm not sure how you're keeping Felix (ie if he's fully free-range, caged w/out time, caged with 24/7 access to a large run area or given his own room) or how you intend to set things up for two separately housed bunnies, but it's
crucial that things appear "fair" from both bunns' perspectives.
A bunny who's keeping count and is displeased with their own score can build up one hell of a grudge over the span of a few months... and they don't stop to recognize if you make up for something after some time is gone past (ie one morning you give a treat to her but not him; that afternoon you do the opposite) or understand when you make up for giving one bunny one thing by giving something different to the other one. If they don't get the same thing either at the same time or back-to-back, it's actually possible for *both* bunnies to feel resentful despite things being 100% fair overall (from an objective viewpoint)! In other words, they keep score like pessimists.
Cages should be the same general size. If they have x-pens, their "runs" need to be the same general size. If one gets free-roam time while the other is caged and has them in view, the second bunny HAS to get an equivalent amount of time (and the first bunny has to be stuck watching them). If you go in to visit one bunny, be sure to visit both. If you offer a treat to one, offer a treat to the other. If you feed one or replenish their hay, make sure to do the same for the other right afterward. Also try to switch up who gets whatever first and who gets "sloppy seconds". You really want to go out of your way to make everything appear equal and fair, because most bunnies will look for any reason to assume that you're playing favorites and they're getting shafted (in particular, the "original" bunny - in your case, Felix).
Never, ever doubt bunnies' ability to keep score, or the potential for their score-keeping tendencies to undermine future bonding attempts! They're like little kids who, after you cut a sammich in order to give them each half, automatically assert that the other child's "half" is larger. Felix is a pampered only child who suddenly has to learn how to share and she's the new kid, who may very well feel she has to prove her worth compared to the original. Like human children, they could take it all in stride (choosing to simply be happy for a new companion) or they could take it as a call to war... and you won't know for sure which is the case until they're either bonded adults or it all blows up in your face...
so assume you'll be dealing with a bad case of "sibling rivalry" - that way you'll be prepared if that's the case and you can be grateful/relieved if you luck out and neither is the "score-keeping" type.
Is it possible that all the precautions about fairness will ultimately be a waste of time because neither bunn bothered to keep score? Yes. *But* the consequences of a bunny displeased with their score have the potential to escalate past a point of no return (ie prompting them to attack the other rabbit at every opportunity) - in my book, that's more than enough reason to be ridiculously cautious.
When I go to serve pellets, hay or veggies, you should see the way Nala (and sometimes Gaz) stare me down, watching to see who's getting stuff first; at times, even pressing their faces against the condo grid or x-pen gate, depending on where they happen to be. If it's Norman, they silently informs me that I'd **** well pony up their share immediately afterward (and why is HE getting food first?? (never mind that they got food first last time)). If I dole out craisins between the two girls, I see them both watching to make sure the other one isn't getting more craisins than they are (making it a pet peeve of mine that it's quite difficult to pull two craisins out of the bag that are approximately the same size, as I don't dare give two small ones to one bunny to make up for giving a large one to the other). The girls watch us like hawks! If one of us feeds the cats, gives the cats treats or feeds the gliders, they stare us down like "why aren't we getting something, too??"
The trickiest thing to display impeccable fairness with would be if he gets fresh veggies every day and she doesn't get any... so it's worth noting that *IF* the breeder you get her from has been allowing her to eat f/v as a baby (by having access to mom's food or w/e), then you can disregard the rule of thumb about not introducing f/v until they're older (12 weeks to six months, depending on who you ask and how well the bunny tolerates them) and continue feeding veggies and such. Whether babies are allowed to eat veggies varies from one breeder to the next; from what I've gathered, it seems that if mom is getting lots of veggies while pregnant/nursing then kits usually tolerate them very well if they're allowed access to them when they start leaving the nest box and nibbling on mom's foods. As a result, breeders who include veggies in their does' diets generally won't bother trying to stop kits from eating them if their kits have never had issues with that. If she's been getting fresh foods, be sure to find out what f/v have been introduced and what quantities she normally gets.
If the breeder doesn't typically give her kits access to f/v, ask her opinion on when to start - chances are, she'll have a good idea whether the rabbits from that line are inclined to tolerate veggies well when they're introduced very early.
Rules of thumb such as when to introduce f/v err on the side of being extra cautious because they're generalizations intended to be applicable to as many people as possible (ie they're as safe to follow if you've purchased a pet store bunny of unknown lineage as they are if you've gotten one from a very knowledgeable breeder because they're based on knowing nothing of the rabbit's history/background). In other words, I encourage you to follow the advice of her breeder even if it conflicts with the guidelines we typically recommend to folks on RO - every rule has exceptions and a breeder knows their own lines better than anyone else. If the breeder says to wait, then wait... but if she gives you the green light to feed veggies right away, feel free to do so as long as they don't appear to upset her system at all.
If the breeder ends up recommending waiting to give her veggies/fruit, there's a chance she won't care since she's not the "original" bunny and is less likely to view the other bunn as a threat (especially if you hand-feed her pellets right before or after serving him fresh foods, pretending they're special). If she does appear to take offense, it may be a good idea to take him to a different room (out of view of her) to feed that stuff.
~~~~~
She is *beyond* precious, by the way - I want to snuggle the crap out of her! I hope you get her and she and Felix end up being the best of friends
.
