Ideally more males than females, but it is extremely difficult to sex them so you really just have to hope for the best. With mine I started getting vague ideas of genders after watching there behavior in my tank for about a month.
I would go with a 15 gallon and a group of 6, you want a group to be able to breed and I think with that may in a 10 they will feel to cramped. They also get territorial when they breed.
Mine bred on their own in my 20 gal tank, breeding them was the easy part. Raising the fry is extremely difficult IMO. I never managed to raise any of them past 10 days. They are very very tiny.
They are bubblenesters, they use the undersides of leaves. They will nest at any level in the tank.
