I wouldn't recommend a pleco in a small beta tank. They are territorial towards other plecos. But mine happily feed with my corys and have no problem. no fighting. The betta and the pleco won't even eat the same food.
The reason I wouldn't put one in there is that even if you find one that will stay very small, like a bristlenose, their bioload is very high for their size. Most algae eaters are anyways.
Betta: Cleo - SD
Other fish: Golden Gourami, Black angel, Clown loach, Corydoras, Danios, bristlenose pleco