I'm pretty sure that Harlequin Rasboras do well with bettas. Try searching on Rasboras--use the advanced search and limit it to the "compatibility" sub-forum for confirmation. Also check Corydoras pygmaeus. It's more of a mid-level swimmer than a bottom dweller so would be different from the other cories you are getting. Also look at Danio margaritatus, also known as Celestial Pearl Danios. They are quite different from zebra danios and I think somebody here has some.
And don't dismiss the amano shrimp! I also really like nerite snails. They cannot reproduce in freshwater, so they don't over populate, but they do sometimes leave eggs on the glass.
. She's got a lot of these species for sale and has an excellent reputation. She has some rasbora species that are more interesting than the harlequins (I feel disloyal to my harlequin rasobra), but they are a bit smaller, so I don't know if they are compatible with betta. If you are interested, ask her and she might know. You'll have fun looking in any case.
One thing to remember is that the platies will have lots of babies! The betta will also eat a lot of the fry, but if you have a lot of plants, some will likely grow up. If you want to be able to keep some of the babies, leave room when you stock the tank!