in one tank ive got 1 male betta 4 sword fish, 5 guppies, 1 pleco, 6 neon tetras, and two angels. Everyone gets along fine, no nipping or anyting, and in the other tank ive got another male betta who simply has to be on his own, hes killled everything that was introduced into the tank. Bettas have very different personalities, some more aggressive than others, it also depends on how big your tank is, don't put your betta with any fish that might nip at his fins if its a very small tank. Ive just been lucky with mine.
I think you need a backup small tank when adding a betta or when adding another couple of fish. Try it and see; you're supposed to be able to but bettas are funny sorts of fish and as said have different personalities. Supposedly though platties and bettas can get along.
Word of warning: your betta might chase your platy at first. They are in the words of Chickadee more chasers and scarers than actual attackers, but don't be alarmed unless things get out of hand. As in the betta won't stop chasing the other fish at all or actually is trying to bite them.
As absolutely wild as it got when I first put the Otos in the tanks and Noel and Marty chased them unmercifully, I never actually saw either of them cause one nip or ragged fin or any type of damage. The chase was on and they made the Otos stay on top of their form and keep one eye open all the time, but it was a "I'm going to show you who runs this show" kind of thing. Then there was the introduction of the 4 little ladies and in one way it was worse as it was all Bettas I was dealing with. The first day there was nipping, but nothing serious. No bloodletting and certainly not any actual injuries. A LOT of chasing. It has been a little over a week now and you would never know that they haven't been together from birth. (of course, like has been said, every Betta is different and they can vary a LOT)