It does depend somewhat on the individual fish but I have tried it and every time, without fail, the guppies (the males) would end up dead and I finaly gave up. Even when I thought it was working out and I had a docile betta, he'd turn around one day and shred them. I just don't think it's worth the risk. Sure, sometimes you'll get lucky and female guppies do work out most of the time and even male guppies kept with female bettas sometimes get along but it isn't worth risking your fish' lives. Don't find out the hard way. There are plenty of alternatives - such as gouramies with guppies or bettas with other livebearers.