How much fish is too much?

[marie]

I don't think going by inches per gallon is a good way to figure out how many fish to have. A lot depends on the system and the type of fish.
You have fish that produce lots of waste and they will reach a relatively large adult size. The sailfin tang alone is supposed to reach 16" when it is full grown and would probably produce a lot more waste then 8 2" chromis (as an example).
All that said I personally think you have too many large fish for a 120g tank. The sailfin, regal and probably the lavender are going to be to big to fit comfortably in a 4ft, 120g even if they were by themselves and I do see nutrient problems in your future if you keep all those fish