Amanos, like most other things we keep in tanks tend to eat anything they can. I used to think they were strictly herbivorous until i had a 15 gal planted tank with a pair of spawning bn and amanos in it. The bn would spawn and I would see new fry all over the tank and then then the fry started to vanish. I know bn parents do not eat their young, this left the amanos. Full size for an amano is in the 2 inch range, they can grab a fry or small fish no problem, and they will. I normally buy amanos 50 -100 at a time every few years.
I have never seen a true SAE eat plants. Perhaps if they are starving they might. SAE are more often misidentified than not.
Bristlenose rasp algae off of things. When this is on finer leaved plants like an amazon sword, the bn ends up rasping off part of the leaf along with the algae.