Usually between 4-6 months, their hormones start coming in and it is possible for them to start fighting, but they may not as well. Other reasons to consider spaying them is that unspayed/unbred females can develop uterine cancer, also the spaying helps improve litterbox habits, and decreases hormonal behavior problems like humping, false pregnancies(building nests and ripping out fur), cage aggression, and also makes their urine less smelly. You can usually get the spaying done right at 4 months, just as the hormones are starting to kick in.
Litter box training just depends on the rabbit. Some take to it right away and some not. Sometimes there are things you have to figure out if they aren't taking to it, like it could just be the kind of litter that you are using that they don't like. I had a bunny that wouldn't use her litter box, I changed the litter to aspen bedding and she loved it and would use her litter box after that. So if you are having problems, just create a new thread about it and you will get people offering different ideas on how to help.
Handling your bunnies alot between now and when your girls get them, will help them with being better with your girls holding them when the time comes. Don't let them get away with nipping now. If they do it, just discipline like their momma would by gentling but firmly pressing their head/shoulder area to the floor for a few seconds. You don't want them to get into the habit of thinking biting or nipping people is ok.
Other things that are good to know for new rabbit owners - feed unlimited plain(no added treats) alfalfa based pellets until about 6 mo. old, then you need to limit their pellets based on weight, and alot of people will also switch their adult rabbits to a timothy based pellet. If you have any poopy bum problems, you may need to limit pellets now. Also, any food changes need to be gradual over a few weeks to give your rabbits digestion time to adjust to the new food. Always feed your rabbits unlimited hay. Some people will feed their bunnies alfalfa hay until they are 6 mo. and then switch to a grass hay. I just feed my babies timothy. Sometimes if fed alfalfa only, babies can get picky about their hay when you try to switch them to grass. So I just feed mine timothy hay and just a little alfalfa. If you have a horse feed store near you, it is the cheapest way to get hay. Just call them and ask if they have bales of timothy or another grass hay. You want good horse quality hay that is green, never been wet, and doesn't have weeds. The bales are about 50 lbs + and cost about $7-20. So much cheaper then petstore bags. Make sure you use a rabbit safe litter like yesterday's news, pine pellets with no additives, aspen bedding, and kaytee soft granule bedding are a few of the ones safe for rabbits. You don't want to feed treats or veggies to your bunnies until 12 weeks old, and then you need to not overdo sugary things like carrots and fruit, and slowly introduce veggies. They need to always have water or they will stop eating, Any toys will need to be rabbit safe as your buns will chew them up. Toilet paper rolls, hard dried pine cones, are cheap fun toys for buns. They love cardboard boxes with crawl holes cut in them. Sometimes it's trial and error finding the kind of toy your buns like. One of mine loves this hanging rabbit chew toy with a bell on the end. It's hung on the side of his cage, and he'll stick his head under it and flips it over his back over and over. None of my other rabbits do this, they will chew on theirs though, but he doesn't, he just plays with his. I can't think of anything else right now. Here are some good veggie lists and how to introduce the veggies, when the time comes.
http://www.rabbit.org/care/veggies.html
http://www.3bunnies.org/feeding.htm#greens
The rabbit.org website has lots of great info for new rabbit owners too.
Good luck with your new buns. They're cuties and I'm sure your girls will absolutely love them. I just love sitting and watching my rabbits play. They'll zip around the room and do their little bunny tricks, which is pretty funny to watch. Oh, if you do let them run around your house, remember rabbits are chewers. Do some bunny proofing, keep an eye on them, and keep electrical cords out of reach because they seem to be especially tempting chewing objects to buns
