Thanks for the additional ideas, everyone. I'm basically torn between going for a small school of colorful midwater swimmers or a small school of bottom feeders/dwellers.
But I am definitely going to have to switch to gravel, because betta boy just spotted and immediately killed the lone MTS in the tank. He didn't try to eat it, just struck it super-fast and super hard, stared at it for a few seconds and then swam away. No point in trying to do sand if he's going to kill MTS on sight.
As much as I would like to try an amano and a couple of nerites with him, I'm not sure that any inverts will be safe with this fellow.
If he's proves to be one of those murderous bettas, I'll have to change plans and segregate him in the 6.6g or 2.5g ... What a pain. Why do we fall in love with difficult fish?!