Chromis shouldn't affect all of the life on the live rock. When I first got mine (I only bought 1 chromis at first), it wouldn't eat the pellets I offered it for a little over a week. I did see it pick at live rock once in a while for sustinance I assume. Now that I have several in my tank, and they readily eat Formula 1 & 2, I rarely see them pick at the live rock.
For your 55, I would porbably get 5-7. Its a good number for a school but not too much so you can have room for other cool fish.
If you want alot of life in your tank you can 1- not get any sort of dragonette because they might eat everything in your tank and 2- get rock rubble and put it in some undiscrete area of the tank (tons and tons of pods
I'm not sure if this is true, but I think that dosing some phyto into the tank usually helps pods
grow. Someone might have to confirm that because I don't want to give you info that is either wrong or just going to make your water foul.