Eel spam, while good, relies on synergy to get going. 2-3 Eels are, weirdly enough, going to be more survivable overall than 4-6. Remember they're relying on their good Df and expending RT's to negate damage, so someone like Maggie who can force a unit to dump all of their tokens (or really just the Empire in general for their PT output) can just mow them down.
As was mentioned above, definitely put it on the board first. There's probably a sweet spot as far as the # of Eels you bring in, but I don't think "use ALL the Eels!" is going to be a winning strategy.