If you have sand substrate in the 10 gallon, pygmy cories would work very nicely - you could easily have 8 or so in there and perhaps a few male endlers too if you wanted. Other options might be a school of microrasbora (eg chili rasbora), CPDs or ember tetras - so you'd be able to have those you mentioned. I'd make a guess that 8 or so wouldn't be too much considering their tiny size. If you went for some of the smaller shoalers, I'd plant it fairly heaviliy as I'd imagine they'd appriciate it what with their small size. There are various options that could work in the 10G. I'm pretty certain that the upside down cats would get too large for a 10G though, I'm afraid.