anything that fits in your bichirs mouth will be gone before you know it,
and I don't think pleco will do well with bichirs, this matter have been discussed over n over again, there are some many examples and similar cases in the forum.
Either small or bigger plecos, they will eventually suck the slime coat of the bichirs, which could lead to bacterial infections, and die...
Either way, plecos have sharp spines all over their body, if the the plecos got swallowed by the bichirs, then there are chances that bichirs will hurt them self, and die...
None of these are meant to scare you or anything, but some people are still persisted with the combination, ignoring that the chances are still there.
cheers