I would not suggest BN plecos, as they get 6 inches long and have bioloads like goldfish.
Pygmy cories are a great choice - they are tiny, with a tiny bioload, and most bettas get on well with bottom dwellers. They are also hardier than the next smallest species of cories (pandas) which are very susceptible to stress.
They do need groups of at least 6, but in a 15 gallon with one betta or even 6 girls in a sorority, that bioload is really not going to be an issue.
Pitbull plecos may also be an option, but they have a much bigger bioload and need groups of 5+.
Pygmy cories are a great choice - they are tiny, with a tiny bioload, and most bettas get on well with bottom dwellers. They are also hardier than the next smallest species of cories (pandas) which are very susceptible to stress.
They do need groups of at least 6, but in a 15 gallon with one betta or even 6 girls in a sorority, that bioload is really not going to be an issue.
Pitbull plecos may also be an option, but they have a much bigger bioload and need groups of 5+.