I have had mellow bettas with guppies and platy. Platy sometimes can be nippy - but ones I seem to find are scared of bigger fish so leave them alone. Guppies, usually do not mind, but there are some mean ones - and some bettas take offense to male guppy colors
You can try. I'd float him, for half an hour, see his reaction (flaring is not good, especially if he does not stop flaring when the half hour is over!), then introduce him. When introducing rearrange ornaments and fake plants. This makes the territory no longer a particular fish's (like one of the platys may "own" a plant). Make sure to keep an eye on everyone! Check fins constantly for ripped edges. Look for stress signs!