I would suggest making sure the divider in the tank isn't transparent. Although some find it entertaining to watch the Beta's flare up at each other it is highly stressful for the fish. Also, Beta's are tropical fish, they should always be kept in a heated tank, Beta's kept in unheated tanks generally don't do much as they are too cold. A Bristlenose pleco would be fine in a 10g with Beta's, also a few corie catfish would be ok, maybe some shrimp?
I would hesitate myself about putting other fish in with a Beta that would share the same water layer (that's just my opinion), they don't like company in general, definately no fin-nippers like tiger barbs! Also, add some plants if you can, Beta's ideally like a densely planted tank with lots of hiding places, Bristlenose pleco's will need some driftwood in the tank also. I wouldn't get Oto's as they're a schooling fish and there won't be room for 4 - 6 of the guys. I would go a 20 long for your plans myself. Good luck with it all.
