I have relatively limited experience with this. I have a betta sorority of 5 with some platies. We've been relatively lucky but are still watching. What I can tell you is that we had one of our bettas kill several live bearers BUT those fish she killed were showing signs of potential weakness/illness beforehand. There has been no other aggression toward any of the live bearers thus far and the betta on betta aggression seems to have worked itself out now. We have had this set-up for nearly a month so are still watching.
Sometimes you will find stores have some in community tanks so you could observe their behaviour there, but also understanding that tanks at the LFS
are more like revolving doors and not really an ideal enviro so the fish may not be showing their true colours. But still worth a look. Otherwise, it's quite difficult to tell until you give it a go. This is not scientific in anyway but my experience has been that red ones are more aggressive. We have two reds and they really are the aggressors. It was a red that killed our weak mollies. Again, that is just my experience and someone may very well have the exact opposite. I have nothing more than my experience to back me up. It all comes down to environment, really. Have lots of places for them all to hide out and enjoy some down time.