I plan to get a pair. If i can get lucky, i'll post a journal for them and add a link here. But that would take some time.
The size of the tank should not affect fish breeding unless the fishes are big and the tank is small. If you want to consider the new fishes then go for at least 50G as the main tank and another tank, probably 20-30G for breeding.
I google-read that leleupis will kill their young/fries if they are breeding for the next batch. However, i also read that those leleupis are good carers of their kin. So as soon as you see the main pair show sign of courting, move them to a the breeding tank. You can transfer them to the main tank once the fries are ready.