Regarding taking the test, I have always filled out the highest tonnage possible (in your case 100) when doing an upgrade then let them decide what to give me.
The tonnage requirements are staggered by tonnage and number of days so you can often get a higher tonnage if you put in enough days. There is no requirement that you are captaining a vessel to get sea time, you just have to be actively involved in the vessel's operation. I actually know a cook on a vessel who has a license but does not know the first thing about actually running it. Pretty scary to me but those are the rules.