As far as the mice issue, it's personal preference. Bigger mice can be meaner and depending on how your snake catches them, they may have a good opportunity to bite. I ususally get frozen/thawed because I can buy 10 or so and keep them in the fridge and thaw one out in some hot water when I need to (if you're not familiar with it, you just stick a mouse in a baggie and some come pre-baggied, into a cup of hot water and let it sit for about 30 minutes at most...then squeeze the baggie a little to see if the mouse is thawed all the way through ie doesn't feel cold in the middle) The only time's I've had a problem with a snake not taking dead vs alive I was able to remedy by running the thawed mouse AGAIN under hot tap water for a few seconds so it'd give off heat for the snake, then having the mouse do a little "hey-I'm-not-really-dead" dance for the snake between some chopsticks (lol)
Again some snakes will be picky anyway- I get all my animals from reptile expos though, and that's one of the things I quiz the breeders on. If the breeder isn't just making stuff up, they'll be able to pretty much tell you the last time each of their hatchlings ate, and when their first shed was. But after a while you will get to see your snake's feeding habits. The one corn snake I have left, I have had for almost 6 years, since he was a hatchling. He was SO picky (as baby corns sometimes are) I had to keep him absolutely undisturbed or he would wander off and forget to eat. It took a LOT of patience and watching him and coaxing him to eat and not get distracted for about 2 years. Now, he goes after anything that moves near his cage.
As for frequency of feedings, again you should learn your snake's habits. Most of my snakes have stayed in hiding until they were ready to eat again, and then at night you could see them roaming their cages for "prey". This would usually happen about a week after I fed them. After your snake grows, if you find them getting 'hungrier' more often, try giving them two mice in a feeding. My adult corn takes two mice every two weeks, and my ball was taking 1-2 mice every week (these are small-medium sized mice about 2 inches long excluding tail). It's also good to simulate a sort of natural feeding cycle- don't always stick to a set schedule. A snake in the wild might find a regular meal and then suddenly have to go without for a period of time. So you might give one mouse, and skip a week, then give one or two mice, and then a week later give another.
Misting depends on where you're keeping them. If you have a dry room, you might need to mist more often. I usually misted once or twice a week, and paid more attention to humidity when she was shedding. Most of the time, with a basking lamp going, it's going to dry out pretty fast anyway, so getting a little rainshower won't hurt as long as the cage isn't constantly like a swamp.
My snake and lizard are with me at my boyfriend's apt now, and we have two cats. One tank sits on a short bookshelf and the other on top of a tall dresser, but the cats love to jump up there when we're not looking to invesitage. Again, thats where cage clips are a necessity, they'll prevent the snake from getting out and the cats from getting in. I've had my small 10-gal tank fall over and the lid stay on only because of those clips.
I havn't had any problems with salmonella. I generally just use regular soap on my hands after I take them out. I got some
wipe-out 3 (basically no-water hand sanitizer for reptiles) but I don't use it very often, just if I'm in a hurry.
I havn't had any fungus problems (just don't keep the cage TOO wet TOO often) and I did have some mysterious little bugs drowning en masse in my corn's water dish, but a good scrubdown of the cage and a bath for the snake in lukewarm water seemed to get rid of whatever they were. I think they may have been hiding dormant as eggs in a bag of substrate I had.
I have not had a lot of experience with keeping multiple snakes in the same cage, mainly because I have not had cages large enough to accommodate two living together. I would exercise caution and keep only a breeding pair together. Always separate during feeding time. Snakes are very solitary and shy in general, you can spend a lot of time making them a nice cage with moss and plants etc and they will sit in their little hiding cave until 3AM and then come out and start climbing on stuff ripping the place apart

Professional breeders end up just keeping their snakes in ventilated tupperware-like containers with heating pads that they store in massive shelving units. The snakes don't mind, all they want is a dark warm spot with unlimited mice lol
Wow sorry I've rambled too much XD I hope my experiences help you out. I have to say ball pythons are awesome snakes and good luck with yours

I'll take some pictures of my two setups here for you soon
1/2" lobes, 8g second lobes, 14g right nipple, 14g vertical labret, 4x 16g rooks. Tongue piercing coming soon...