The impeller is something you want to be inspecting at least once a year, so you don't want to do anything that makes it harder to access. One of the most useful engine add-ons is a
"permanant" removable impeller cover. So, for sure you do not want to use anything on the impeller cover that hardens or is hard to remove & reseal.
For the impeller cover, i would just go with the gasket, and maybe a bit of marine grease. For the thermostat, also grease, or maybe some non-hardening gasket sealant like
Permatex #2.
I don't have alot of experience with the RTV gasket stuff, but I don't think I'd use it in either of the above applications.