i've kept different fish with my male betta, including platies, and they all got along good, but some male bettas are different, they have different attitudes, so what u can do is try putting platies in, and if they dont get along, i'd either move the platies or the betta
i think guppies are the worst to keep with a betta, only cuz my female betta ripped my male guppy's tail to pieces and its not a good thing to watch, im sure male bettas would do the same, i've read all over the internet that its ok to keep guppy's in with betta's but i dont think it is.