Yes plecos get huge. Bigger than or as big as clown loaches. I have a 55 gallon fish tank, a 10 gallon fish tank, and a 2.5 gallon fish tank.
In the 55 gallon tank I have:
3 goldfish, about the same size as my middle clown loach
3 clown loaches, there is a 4 inch, a 6 inch, and a 7 inch
2 plecos, not sure the size but I think about 7-8 inches each
All huge
In the 10 gallon I have
1 black skirt fish
1 male, doubletail betta
In the 2.5 gallon I have
1 male, halfmoon betta
I want to get some more fish like some tetras and put them in the 10 gallon. I also want to get a 5 gallon with the full hood cover so that I can get some hatchet fish, they jump so I would need the tank fully covered.
Male bettas will fight if they are put in the same tank. I would think that any male fish would fight. Bettas are solitary fish, they don't really get along with other fish.
You can get cory fish, they are bottom feeders and they can keep the tank clean. They get 2 inches and are great for ten gallon fish tanks.