How to determine tankmate suitability?

I just got my betta last Friday. He's living in a planted 10 gal, and I'd like to get some tank mates to fill out the tank a bit more. I'm thinking a school of 6ish ember tetras to go with him since they seem a bit less fragile than the microrasboras. However, I don't really want to drop money on what may end up expensive feeder fish, so is there any way to tell if my boy is going to be fine with other fish, or if he's going to go on a killing rampage?

He's a white twin-tail, currently living with a mystery snail that he entirely ignores.