I must agree with what has been said before, the first species of fish I thought of was one of the puffers. In a 15G you'd be limited to the smaller species of puffer, your best bet probably being the dwarf puffer as it's more readily available and would fit a tank that size best.
As for how many you can keep, I personally wouldn't recommend more than 1 through bad experience myself. Others will tell you you can keep 3, one for every 5 gallons. If you do get 3 it's unlikely you'll get them old enough to sex, but if you can try and get all females or 2 females 1 male.
They can be sensitive little fish so a mature tank would be better for them. As we know this isn't the case I'd recommend cloning one of your other tanks rather than cycling from scratch. Also make sure there's plenty of stuff going on in the tank to break up their sight so they can get away from each other and hide. A more natural environment I'm sure would be preferred.
Like with all fish, please do read up on them first before you make up your mind and buy them =)