How many platies did you get? How many guppies did you get?
The number of platies will affect whether returning your red one is a good idea.
'Time outs' work just about as well with bettas as they would with a rock. As smart as bettas are, a few minutes of seperation won't do anything. You have to remember this is something new for him and he needs time to adjust. Keep him in the breeder trap for a bit if he's still being agressive and give him time to get used to seeing but not touching/hurting the others.
Since you decided not to quarentine the fish, keep an super tight eye out for sick fish or abnormal symptoms. There are a lot of pathogens that can be carried in on outside fish, which can end up with the whole tank dead. If you notice any sickly fish, remove them at once and quarentine them (again, if you don't have the qt equpiment, it's always a good idea to invest in it should you have the funds)