I have kept both guppies and mollies in full concentration SW. Platys no. Not saying it can't be done, just that I never tried it nor had a reason to do so.
When I would acclimate either the mollies or the guppies, I did it over a 24 hr period. Never had any issues with mollies. Guppies went okay as long as the females weren't close to giving birth. If they were, I found that a small percentage of time they would release the fry prematurely.
