Barbs I think are a good idea, the eel is completely marine so I don't think it would work with just a brackish set up, depending on how salted your water is. Rainbow fish like to be in groups of 5, so I don't know if that would be good in a 55 gallon with so many different fish unless they were dwarf rainbows, but I don't know that for sure considering I suck at imagining dimensions in my mind. Rainbow shark can be very aggressive from what I understand though if you have enough space and the right tank mates it should be ok.
40 gallon with 13 hermit crabs- 11 PPs, 1 E, 1 Straw
~2 Dwarf Puffer- Sidney and Cairo, 4 Amano Shrimp, 4 Blue Pearl Shrimp, 4 Yellow Cherry Shrimp, 2 Mandarin Cherry Shrimp, Too many RCS, 4 Crystal Red Shrimp, 4- Otos
~White/Teal/Red Betta- Phantom, Snails
~Leopard Gecko- Desert Rose