I wouldn't keep a betta with angelfish- not because I don't think you can keep a betta in a community tank, but because I think angelfish will be too aggressive for it. As far as recommending a single fish, I suggest a dwarf cichlid like a ram, krib, or apistogramma. At that point, I'd call it stocked. A 29 gallon is a bare minimum for 2 angels IMO
. I've you've ever kept an angel that grew all the way to adult size, you'll know what I mean.