Hi there,
I don't know much about PC lighting, so I can't comment about the light-dependent aspects of the question (corals). But for fish, it really depends on who you talk to. Ways to look at it: 1. based on inches of fish per gallon, 2. maximum amount you can put in that is supported by your existing biofilter media, and 3. based on the type of fish (e.g. tangs, chromis, and gobies have different swimming characteristics and needs, which depends on tank size, amount of LR and sand, etc). There may be more, but I don't claim to be an expert at any of these ways, but just thought I'd share.
kris