Guppies generally don't need too much of space, but yes... I believe these might need about the same, since they will probably be growing still

hmmm .... Since they are schooling fish ... I'd say around 6-8 ... I may be wrong since its usually an inch of fish per gallon of water but even full grown I've never seen a problem. Breeders will keep 30 in a ten gallon ... Be careful though as they breed like crazy. Try to keep the temp a little low or get all one gender. Or divide the tank males from females unless you have plans for the babies. Though they will eat the babies if you dont seperate them

A guppy needs about 1 liter of water per cm of fish (they grow to be 5cm). I'll try calculating fast!
You could fit about 4-5 fish. But then buy only males, because such a small tank can't handle all the fry that guppies produce. Males are easily recognized on their colorful bodies and big tails. (And also the gonopodium on their anal fin)

Not at all At most they would a bit if you wouldn't keep enough females with them

But on the other hand, these fish are small, and they don't create much waste, so if you wanted to, you could grab a few females too, but then think two females per one male. So at most you could have two such couples, in a total of six fish. They don't live too long, so it might be cheaper letting them reproduce on their own (they would eat most of the fry anyway. And as long as you'd stick to weekly (no less!) water changes while keeping an eye on their water quality But yes, final statement would be about 5-6 fish