I've always heard that bettas with anything "finny" can be a dangerous mix. Meaning, if a betta can look at it and say, "Wow, ornate fins... wait, like mine! I must kill the competition!" then it's a bad idea. Of course, I am certain someone that has a betta with angels will speak up shortly and extoll the virtues of this type of setup. Individual experiences vary. You could always set the betta in a small bowl next to said angel tank and see if he flares.