The swordtails will interbreed with your platy too. Your male swordtail makes no difference between a platy and swordtail. Also, swordtail males are really aggressive towards other males and can kill each other so just one is best. And generally, swordtails grow big enough and are not recommended for a 15G tank. They are very active fish and like swimming space. If you are going to breed them and intend to start with 8 adults, you'll be overstocked in a couple of months.
So 15gal with 8 adults is not recommended? 2 males and 6 females but right not it is stocked with 1 male and 4 females.
I won't worry with increasing numbers of my swordies regarding to fry, I have one empty 28gal i think it's enough to hold the first few batch of fry then if it still increases most will go to an available LFS.
It's not impossible to keep 8 swordtails in this tank but it is not recommended. They grow up to 3.5 inches which is 28 inch of fish. If you go by the rule of one inch per gallon, you should have no more than 15 inch of fish which is just nearly your current 5 fish. The only point I'd make is that swordtails are very active fish, so they won't do well overcrowded.And I would have just one male for all the females. I don't think you'll get away with two males in a tank that size, one will eventually "eliminate" the competition.
The females can give birht eight times after the last pairing with a male so 4 females, eight times each, between 20-100 fry each time possibly is a lot of fry even if you get lucky and most get eaten. Then if you keep them long enough for the males to mature so you can at least sex them, they'll start fighting.