10 gallons is perfect for a betta. They should never be kept in with a normal community tank anyway so the sooner you get him out of there the better.
Since he clearly attacks midwater fish, then I would suggest you stay away from those completely and go for small bottom feeding fish instead. You will need to have a sand base for these fish, as gravel wears down their delicate barbels on the mouth. Argos Play pit sand is cheap and safe after you've given it a good wash, or you can buy proper aquarium sand from your fish store ( this will cost more)
5 panda cories or peppered cories would work, OR 8-10 pygmy cories, OR 5-6 khulie loaches.
Bigger shrimp such as Ghost shrimp should be ok, and ADULT cherry shrimp should also be fine, though babies may get eaten but that's ok, it'll keep the population down if he does. Shrimp also add almost nothing to the waste prodyuced in the tank so they are always a good coice of companion, and are active enough to be interesting.
Apple snails are nice as well. One would suffice for your 10 gallon.
Make sure you put in a LOT of live or silk plants (never plastic) as bettas like to have plenty of vegetation to hide in. The plants should also be mostly made up of tall/long varieties wether silk or real, so he can rest on them .
Your filter should have an adjustable flow, and the flow should be turned right down on low so he doesn't get blown about in the tank by the current. Sponge filters are good, the Rena Filstar internal cannister filter is excellent too. Hang on the back types of filters are also quite good as well.
