As I mentioned in your other post, Ageneiosus brevifilis is a synonym for Ageneiosis inermis. The data page for this fish over on PlanetCatfish states it is known to reach at least 21", and there are some pictures of several large specimens. I think a picture would help to confirm you do have the fish you think you do.
HTH
Martin
Should be fine - it has probably already got rid of it from one end or the other by now. If it keeps doing it, you might need to worry, but as a one off, am sure fish can deal with the odd piece of gravel.
HTH
Martin