Poklei, you have to realize there are over hundreds of plecos. Many of them grow rather large and bulky so I would not consider that in general, plecos will easily be eaten by bichirs. If you think your pleco may be in danger of being eaten, take it out now before you regret losing it. It's actually a good thing you asked. Plecos have serrated spines so it's not surprising that most fish die from swallowing catfishes. Therein lies another issue with most plecos sucking the slime coat of most tankmates. It's all basically trial and error and logic.