Add water? Seriously, theres a reason they call guppies'millions fish' I'd say... 10g? Just try and vary where you get the parents from and don't breed siblings together to create stronger offspring.
A breeder tank? I'd say you could have a 10 gallon with maybe 1 male and 3 females, and perhaps some other small fish, like Pgymy Cories or maybe a Dwarf Gourami. Or you could have 2 males and 6 females, alone, no other fish. The main thing you have to consider though, is the fry. Guppies have tons of fry, and all those fry need homes, or places to go. You could easily have a couple hundred fry in a few months. So just think it all through before you start breeding.