I think you should just complete your school of pygmy cories and get a nerite snail for algae cleaning. Cories, neons, and platies all need to live in groups of six or more, and there's just not enough room for two whole schools of fish to live in this tank along with the betta. Minnow type fish like tetras and danios need lots of swimming space due to their very high activity level. Personally, I would not cram them into a 10G tank.
I would just slowly add cories until your school has about 6-8 individuals and then stop there. Your betta is so beautiful that you don't need a bunch of other little fish stressing him out and nipping his fins to pieces.