You could do bettas with ottos. From what other people have said (I haven't had any ottos myself) it seems that they will pretty much just stay out of the way. Just make sure that you have a good amount of algae growing in there because they can be fussy eaters. I've heard that some accept romane lettuce, so you could try to supplement with that.
Wow, that's a lot of bettas :) Have fun with it! I'm looking foward to the pics ;)