Swordtails And Platies

[Ricapath]

Hi.
A while back i bought 3 male swordtails but one died and my mum replaced him with a female swordtail so the the remaining male became aggressive towards each other and one eventually died. Now i have a female platy and i was wondering because i read this somewhere which may or may not be true but swordtails and platies can mate so my questions are: Will my swordtail be agressive to a male swordtail or platy if introduced? How many female swordtails or platies will I need for him not to be if I want another male swordtail?
 
The tank is about 75 liters, 20 US gallons, 16 imperial gallons and has been running for about 2 years. i have a fluval U2 filter. 
I have: 
1 swordtail (m)
1 platy (f)
2 golden gouramis (m&f)
5 giant danios
3 scissortail rasboras
3 black widow tetras
1 plec
3 albino corydoras

 

[RyanTheFishGuy64]

Platies and swordtail WILL mate and are almost never agressive towards any other fish.
On the other hand, you have ALL of those fish in a 20g? thats about 3 times as much as you should have IMHO

 

[DreamertK]

I'd get a 30 gallon or larger tank and move the male gourami to the tank. Add 3+ rasboras, get rid of the black widow tetras since they'll nip at the gourami, and add 3+ albino cories.