I have a tire track with my 3 fogos, 3 FL gar, 2 p. gibbiceps plecos and a marble achara (all over 16")
so far the only aggression I've had is between the bass.
I kept my old tire track with kelberi, monoculus, orino, ocellaris and had similarly little issues.
Pretty much as long as the eel is big enough to not be slurped up, which is usually about the same length as the bass, you should be okay