I think all females are good, if they are large enough to sex. The rule of thumb is 5g for the 1st DP & then 3g for every one after. That's if all you keep are DPs. You could go with 2 males & 4 females. I think it would make for a more interesting tank to have both sexes. All males will fight to the death, possibly resulting in no puffers left. A group of ottos & also, cherry shrimp will do well in there.
