Actually, you could destroy Ella using regular firepower after choosing to destroy Ella instead of pass by it, which would let you save the FP for the Vesuvius encounter. It just takes a good while to whack the station with conventional weapons, even using a Dragon's torp and all its missiles (or at least the ones not spent during the rest of the engagement; I don't recommend trying to destroy Ella while hostiles are still present, as it takes even longer that way).