Is this stocking ok?
 

Current Stock in 20g long:
6 emerald green cories
2 female bettas

I was thinking about adding 2 boesemani rainbows or some guppies or some neon tetras? What would work?

Sounds fine to me.
I would avoid the boesemani rainbows.
They do better in a school and grow too large for a 20gal.
I think a group of guppies or the noen tetras would work.

Guppies would be fine in there. If you are worried about too many babies, you could always just get male guppies.

The cories are good natured enough to not bother most species, bottom dwelling to stay out of most other fishes way, and tough enough to shrug off all but the most pissed off other fish.

What you can do with the tank though is largely going to depend on the temperament of those female bettas. Some won't care about the other fish in the tank, some will get as aggressive as any male betta. Guppies and neons should be fine, both can dart away pretty quick and will keep to themselves. I'd just watch carefully that any male guppies don't get harrassed by the bettas. It's unlikely, but so is three shrimp crawling up a heater cord and out the only hole in the top of the tank in order to jump to their deaths.

The rainbows, they'll get on the big side for a 20 gallon tank.