Fire, TT, spiney ect will eat what fits in their mouths.
I've had Eels that make real good buddies with my bettas and other similar sized fish. I had a very sick betta and my TT stayed by his side the whole time, amazing thing to watch they were both fascinated with each other. But he'd munch a neon or Guppy in a heart beat.
Love Eels, very intelligent and great characters.