I have a betta male and female in my 55 with 3 serpae tetras and I have 0 problems. Been that way for about 3 months. I wouldn't advise anyone to do it, but if you watch for problems you can just pull your bettas out. I also have bolivian ram pair, german ram pair, blood parrot pair, several cats, plecos, raphael, 2 flounder, African leaf fish pair, clown loach school, and a ramshorn snail.
YES, I know this is a crazy mix... but they are happy, and healthy, and it works!!! Many are young and will be separated with age, but I don't think the serpaes will bother the bettas as long as you have enough room, hides, and perhaps the cardinals, and phantoms will keep them uninterested in the bettas!! You can only know by trying. You could put the betta in and keep him in the net and see how the others react, any aggression, don't put him in.