Males can (and ime will) get into territory disputes that can be deadly. I had to separate mine (that I was told were littermates, which can make shared housing easier, but apparently they weren't) after a couple of weeks. Better to separate now if you can, although the theory is that in a big enough tank/cage, they could each claim a corner and be okay. I'd probably use at least a 20 gallon for that, maybe even a 40 if I had it. (I had one absolute sweetheart and one I named Grendel for exactly the reason you'd think!) Males just aren't as social with each other as females, but their social skills also seem to come in part from where they're bred. On international mouse owner forums I've visited, it's been noted that the trend among American mice is territoriality whereas mice in Oz and the UK seem to be of a milder temperament over all. Once they hit sexual maturity, though, it's a crap shoot. I wouldn't risk it. You never see injured/dead ones in the cage at the store because those are generally pulled out to be feeders for other animals. A 10 gallon "starter" tank is plenty of room for a singleton, but a 5 gallon can work if he has stuff to do.
I never bothered with feeding alfalfa pellets because they're not really herbivores. They seem to like loose hay as bedding, in addition to shredding paper towels/kleenex. I gave mine scraps of fleece to drag around as bedding too. They can get cold easily, esp. if the room is cool for a chin. Generally I modified the Sue Bee's diet (it'll pop right up from a Google search) and put in about 2 Tbs of it in every day, then gave steamed/fresh foods or whatever we were eating like many people do with rats. I usually used a low protein dog kibble instead of the lab blocks, but either one works. The trick is not overfeeding any one thing and making sure they get plenty of exercise. For training purposes, you can put a teeny little dab of strawberry jam, peanut butter, yogurt, or applesauce on your hand and put it in the tank. They'll come investigate you to see where the good smelling food stuff is.
Speaking of exercise, I distrust that type of wheel because of the spokes. Just like with chins and hedgies, they can get feet/nails/tails stuck in it. There's a type of mostly solid wheel called a Silent Spinner that I liked a lot, but you'll have to get the 6.5" recommended for Syrian hamsters rather than the littlest size. They need room for tails and a full extension of their back and legs. There's also a plastic take on the chinchilla Flying Saucer that worked pretty well if you get the medium sized one. Thankfully, plastic chewing isn't the problem with mice that it can be with chins. And, mice LOVE to climb and be athletic. You've got a great set up for the most part, but a few "condos" out of Pop-tart boxes, tissue boxes, paper towel tubes-- anything paperboard like that will be a huge hit. I also braided strips of fleece and used floral/jewelry wire to hang them from the mesh top of the tank for climbing purposes. The biter calmed down considerably with his "jungle gym" in place.
I dont know of any mice forums but let me know if you find any I am just curious to learn about them as well.
