Most of the time, Platies make good tankmates for Bettas. I've kept males and females with Platies without any problems but, as always, it depends on the personality of your Betta.
Danios are more iffy. Sometimes Danios can be a little nippy, sometimes they are alright. With a school of 6, you would probably be alright but I would watch your fish very, very carefully to make sure he isn't getting nipped.
A store near me keeps a male in every peaceful tank they have. Their tanks are about 20g and the males in the Danio tanks appear to be alright.
There is always the posiblity that you have a particuarly aggressive or territoral Betta that might not accept tankmates though.