Plecos generally don't eat much of anything that isn't green. So your frog should be very safe with one. Most of the time, plecos don't bother anything, so your betta should do okay with one. The only thing I'm worried about is that once in a while, you can get a bad tempered pleco, and then I wouldn't be so sure about your betta... but it should be okay. Bettas usually only ever bother other bettas, too, so I don't think you have to worry about it being aggressive, either.