Honestly, if they nip each other, they may be more likely to nip the betta.
If you go for a plakat, you won't have the flowing fins, but you may also wind up with a more aggressive-to-other-fish type than a super delta/half moon type might be.
One of the many example of good sense above included having a back-up plan.
If you already have a cycled 5 or 10 gallon ready to take the betta in case of trouble, maybe he might just as well go in the ready, separate tank.
I have bettas I'll put in with other fish, and some I won't.
I have fish I'd put in with bettas, and others I wouldn't.
And every time I have put a betta in with other fish, I've known that betta might have to be pulled out, maybe later that day, week, month.
And that's one of the many reasons we ALWAYS need MORE TANKS!
Great excuse, huh?
Edit: one of these days I'll find all my typos before I post and die of shock before discovering I've merely missed them completely.