For fish, the general rule is 1 gallon per fish. Also, sword tails like to be kept together, so I would get at least three. If you are worried about fry, the parents should eat most if not all of them.
Depends on the size of the fish. I think 3 swordtails and a pleco will be just fine. You have to be careful about the type of pleco you choose. I have one that is over 12 inches. Some of them will stay much smaller. Maybe you could get 3 swordtails and a big snail. I have an apple snail that is much more interesting to watch than a pleco.