Hmm....well in a divided 10, I think your options are pretty limited.. basically, your kind of stocking two 5 gallons here. I wouldn't go with corys, I think they need a little more room...a few shrimp on each side would be neat, but it depends on the betta whether they'd snack on them or not. I think ghost shrimp are pretty cheap, so you could always toss a few in there and see what happens. Some Ottos on each side would be okay, since there small, but I wouldn't go with those unless you have a serious algae problem. An ADF per side is another option, though I hear they're pretty hard to feed....
My Supernova flares at absolutely everything too, though its more of a 'Look at me! I'm over here!' sort of thing...however, if yours is more of an aggressive flare then he might be a little harder on tank mates.