Overstocked?

Sounds fine to me over time.

yeah thats totally fine

depending on how much coral/rock etc you have you even have a bit more room

sounds fine as long as there is plenty of swimming room for the tang, they really like to zoom around the tank.

Explain how coral(living?)Rock effect total bioload

It doesn't have much effect on bio-load, what is does do though is limit the available swimming room for fish. Tangs like more open tanks with lots of room to swim, so having a 100G tank may be ok sounding, if you put 250lbs of live rock in there, then it doesn't sound so good for a tang. Also, I have seen people with lots of SPS corals that really filled in there tank, forcing them to have to get rid of their tangs because the corals blocked a lot of the swimming room for them.

Sounds like a good stock list though. Personally I wouldn't go with a tang in that size tank, my personal belief is 150G is minimum to make them happy, but I know it can be done in a 100G, especially a long 100G.