well, armatus might eat actively swimming that fit in their mouths. ive seen 50cm armatus eat 6cm carp. carp are pretty thick and high fish so...
never seen one eat a small pleco or clown loach tho.
Depends on the fish, I have a peacock eel and at the store, he was always in the gravel. I took him home and he was swimming all over the place and in and out of the river rock caves I made. I was at the hospital for two weeks for the new arrival of our baby girl and when I got home, I have only seen him a handful of times because he is always burried in the sand.
my fire eel has never buried in the gravel i think if you provide them with a pipe or something to hide in they will use that instead and it stops them getting scratched up!