Well, if the eel thinks it can fit it in its mouth, it will try. Personally, I would be quite hesitant, as the angel doesn't quite have the same tools for defense as something like a dwarf lion might, or even a tang (but I know of no tang that would perform well in a system of that size--it would have to start very small and if it performed well it would soon outgrow the sys).
I'll suggest you decide which animal you want more. The eel will be much more restrictive, in terms of stocking buddies, whereas with the angel you would simply have to avoid other pygmies, but could add basslets, grammas, hawks, etc.
I also don't normally recommend folks go with animals that I know are wont to outgrow the system in a relatively short period of time, thusly I cannot recommend any puffers or triggers, not even the more docile ones.
I have to think about this--a Foxface would do fine with the eel, but they can be pigs and would soon outgrow the tank. Same thing with Pomacanthids. Filefishes might be a consideration, but having been doinked by the unpredictable Snowflake, I can't give them the "seal of approval". And that's the big problem with Snowflakes, they are unpredictable. I've known so many people, more than I can recount, who had a group of residents together for months or even years, and one day the Snowflake just "turns", and eats someone up.
Others that would house respectably with a Snowflake would be many of the larger wrasses (the Coris gaimard is a personal fave), but they would only do so well for so long in a 55.
Sorry I can't be of more help here, if I had me a "bible" I'd be able to better jog my memory, maybe there's someone else who has other ideas on it. Or, you can reconsider the Snowflake itself.