In a 75 gallon tank, sometimes you need more than 6 thorichthys to keep one fish from taking over.
I would start off with 12 juveniles.
I started with 12 thorichthys in this set up
I never had to separate any fish, they all got on fine for about 3 years.
Eventually I moved them over to my 210gallon tank. They started to act much more aggressive with each other than they ever did in the smaller tank.
Years ago I put 6 meeki in my 210gallon and it was like a war zone. My dominant male became a real pain to the other meeki. I added another 14 meeki and all was calm again.
I'm certainly not a believer of over crowding fish tanks. In my experience thorichthys will be at their best in decent numbers. I don't mean cramped in and over stocked. Just enough individuals to dilute aggression. I have found over the years that 6 is not always enough.
Regards to swordtails. The ones I had were rio otapa swords.
They are a beautiful xiphophorus helleri variant and I highly recommend them if you can find them. Agree with the others keep one male to three females. Multiple males can work if you have more than say 5 and plenty of females to go round. If you only have 2 or 3 males they will probably kill each other.
Rio otapa swords
