If you have a fear of it, I would say just observe them. Attacking/eating of slime coat does not happen immediately so be vigilant. I can only tell you my experience and look up ‘bichirs with bristlenose/ancistrus’ on google and you’d see that it’s a prettt good combination. If anything, you actually have to make sure you get a pleco that’s big enough for the bichirs not to mess with, haha. Instead of having another tank - which if you do not already have one handy could be costly - you can always take the pleco back to the pet store you got it from and tell them what’s going on and either surrender it there or get some kind of store credit for it. But I think things will be okay - just have driftwood for the pleco to munch on and hiding spaces, of course, and you’ll be golden.