I'm sure your combination has a high chance of success
(chance of failure really depends on your betta's attitude,some are more aggressive than others)
I've been obsessed with bettas and IME they are not really that aggressive.
Only to those that resembles or irritates them.
I've had bettas with cory's and shrimps and had another with danios and tetras.
And IMO, you could even add more fish
(just no more male bettas, or other uncompatible fish)
Or you could try a betta sorority
that 30 gallon would be a total paradise for the betta, and with that tank size I can say you are understocked
with the 15 gallon though, I think you can add another tetra or shrimp. But, it really depends on your filter,decorations,hiding places, or plants.
Oh, and try adding the betta last. I've read somewhere that it somehow lessens aggression since the betta wouldn't be claiming a territory with the presence of another fish that got there first (unless your acquired betta is a total bully.hehe!)
Well, it worked for me
All in all, i think you're good to go after you cycle the tank
(CYCLING = VERY IMPORTANT)