Any suggestion that a Betta splendens cannot go into a tank with xyz occupants sets me to thinking about the companion fish. If those companions are aggressive or territorial, the Betta will become a victim. If those are peaceful fish, the Betta will be an upstanding member of the community. Although I specialize in livebearers, I find a Betta splendens is never a threat to my other fish, although sometimes the more active or aggressive fish can be a threat to him. Guppies and platies do not fit that mold of aggressive fish in most cases so the Betta should be fine with them.
What I do not see being discussed here is how mature your tank may be. A newly established tank that has not had time to develop a bacterial colony to process nitrogen wastes from your fish should never have any, let me repeat that never add any, new fish until the filter in that tank is fully developed (we call that cycling).
If you have not already explored the cycling subject here on TFF, please use the links in my signature area to help you understand what we are going on about when it comes to cycling.