I became aware that a 500 grt Inland also certifies you to Master 100-200 grt vessels. Do/can you get a separate endorsement for this or is it understood?
It does not slow you to be master on vessels 100-200 grt, itallows you to be master on vessels “less than” 100 grt (note that a 100 ton master is of vessels “not more than” 100grt). It will be restricted just like your 500 ton, so it will be inland.
I expect the NMC will not put it on your license but that equivalency is in the CFRs so it is not disputable whether it is true or not. If you worry, print a copy of that page of the CFR and be sure the full citation is there (46 CFR 11.xxx) and carry it with your documents.