My eel is about 18-20" I was feeding him one shimp every other day. I increased it because of the Angelfish. These are the small salad shrimp, not larger cocktail shrimp. I do think its too much also, and I'm weening him down to two every other day.
The "bad eel" as my daughter says, would go find and eat the angelfish food before the angel could get to it, once the angel went into the eel cave to try and get his dinner back, but it was too late.
If I feed the eel two shrimp, then put the Angel's food in the eel will usually leave it alone. Although the other day he had his two shrimp, then half the Angel's block before the Angel took it back.