marine beta

Dont know what and how small your dotty back is but never had any that wasnt cunning or fast enough to live with a Comet, dont ask me why.
Although will eat small fish but rather much smaller mouth proportionate to its body size when comparared to similar size groupers, lions, etc, etc

I would be more concerned with shrimp's future but again depends on your aquascaping/kind of shrimps as well as their alertness.

I had kept WELL-FED Betta in reef with few species of ornamental shrimps as fish usually stay/just hung around near top in the rear, behind the rocks most of time other than feeding time.

If willing to take a chance, try finding one that is weaned off live food.

btw, how aggressive or should I say territorial, is your lone yellow tang. Ther is a chance tail of Betta may be nipped at in the beginning depending on your exisitng fish's temperaments.

btw, it can go for carpet diving so tighten up any opening especially in the back.

Good Luck but DO NOT underestimate what fish are capable of or how powerful their suction is. Even if it cant swallow, can do enough damages to immobilize/cripple the shrimps. Ive seen how seahorses have swallowed the shrimps which I thought were impossible for them to swallow.