It's possible it might work, I've done it before, but there are some things to worry about. The Betta may see the Platies as a threat and terrorize them, and possibly kill them. Or the Platies could nip the fins of the Betta. It depends on the temperments of both fish. You could try it, but you need to watch them closely, and have an alternate place for the Betta. Try it in a nice biggish tank with hiding places too.
I put him in with the Platies for about 5mins and watched what they all did, But i didnt wanna chance anythink going wrong so i took him out and but him in his own bowl.
Yeah, that's probably safest for both fish. If you want to get the Betta some friends, you could get a small tank and get an ADF, or depending on the size, some Cories. Though they really are fine alone.