The setup you described is probably intended for a 90 or 110 gal
tank. They are pretty deep and need 400w MH
bulbs. I think it might be a bit overkill for a 55 though. We have a couple of folks here who are using a 2x250w MH
setup on 55 gal
tanks with good results.
As far as the 6 54w T-5's being enough light for a 55....the answer is maybe. If you're going to keep corals with low light requirements...yes it should be enough. You might even keep some that have medium light requirements as long as they are close to the surface. It won't be enough for high light inverts though. No clams, anemones, SPS
corals, ect... For them, you need MH