There can be problems with only keeping two of each. If you have a male and female of each, the male will likely harass the female all the time. If you have two males of each, the stronger on will likely dominate the other. Same could happen if you have two of each with females as well. With only two of each species there isn't enough members to spread about the aggression (and they seem to like being in a group anyway). One of my aquariums is a 31 gallon male platy only tank, males only so I don't get over loaded with fry. At first I had five, it worked well for a while until one clearly became the top guy and dominated the rest. Now I have eight and it works much better. I don't know if I want to add a few more and a have a nice group of twelve or so, or if I want to add a group of zebra danios. Anyway, if you tried your two of each, I would go with something with at least a 36" length because the mollies (swordtails too) get quite large. Post back for more details if you want to try a bigger group.