I think you could get a group of female JD's to 3-4" in a 30 gallon no problem. Keep water clean and they will grow quickly, seems to be the number 1 factor. This would be a good idea, since there is no guarantee the fish will like each other if you just plop a random female in with your male. If you grow out 4 females, then put them all in and let them pair up (and remove the rest) you would have better chances.
As far as the divider, yes many people use this method, I have used it myself many times. You just need a male that is significantly larger, so that the female can fit through a hole (or two) in the divider that is just the right size.
As far as the divider, yes many people use this method, I have used it myself many times. You just need a male that is significantly larger, so that the female can fit through a hole (or two) in the divider that is just the right size.