I have a Bristlenose Pleco in with one of my Bettas and they seem to get along alright. The Betta will chase the Bristlenose every once in awhile but there are good hiding places for him so it works out well. I have to say, though, it really depends on the Betta as I had a different one in with the Pleco and the Betta seemed to make it his mission to chase the poor little guy constantly. I've been told as a general rule a Betta is more likely to accept a tankmate the younger he is.