I've kept up to 1.6 in a ten gallon with a cleaning regimen of twice a week. No problem.
As for freezing.... as long as the mouse is naked (baby) you can freeze it. If it's a fuzzy, put it (or them) in the freezer bag, give the bag a good whack on a flat surface, then put them in the freezer. Once they are hoppers you can gas them (NOT before.... baby mice are resistant to hypoxia and it takes longer to die if you gas them then if you freeze them!)
To gas my mice I get a large plastic container with 18 inch sides. In that I place a shallow container with water and dry ice. I let that sit for five or six minutes and then in a small cage I place the animals at the bottom of the larger container (NOT on the ice) .... I put the cage sideways so that the air holes in the top are at floor level. They jump around for ten to fifteen seconds, and then collapse. Leave them in for five minutes so they suffocate, and you are done. I would never place a live furry critter in the freezer, just not humane.