Bettas not only can attack other fish, they can be the victim due to their long fins. There's no reason to try it, imo.
Also, the 6 in a group is just a minimum number. More is obviously better.
Not to be offensive here, but why not add a betta? They're incredibly hardy if you know how to take care of them, they display beautifully once acclimated, and they make wonderful showpiece fish. People keep gouramis, angelfish, and other species specifically bred for long fins in community tanks, and those fish are just fine sometimes.
Ehh, 6 might be a good minimum for others, but I advocate for 8.