I would assume that a 1989-'91 100Q/200Q gas tank would interchange with a 5000Q but is there a quattro/non quattro difference in the tank?

Yes. AFAIK all the gas tanks in Type 44 quattro cars are significantly different than gas tanks in non-quattro Type 44 cars.

That is correct. You can change tanks between quattro versions of 5k/200/S-car (92-95.5) as far as fitting/attaching to the car body is concerned.

However, the internal anchorages for the pump vary. If you have a 200 that uses the "basket" type retainer for the pump, you cannot use the basket in an S-car tank. You have to exchange the larger retainer for the basket from the 200. If you have a 5ktq with the flexible triangle attached to the pump with a hose clamp, you need to replace the anchorages from either a 200 or S-car in order to install the 5K pump.

I found an '89 100 in a yard that cuts & crushes, I had the chance to strip it before it was gone, my 5KQ tank was leaking some years ago, I removed & patched it but always worry it may corrode further. I removed many small parts that are now in use on both my 100Q & 5KQ. What is AFAIK?