I don't think there currently is a way to select units by rank, but I really wish there was. When I'm advancing to a building I want to capture and there's limited opposition I want my elite units to capture and feed XP to the rest. Selecting each type of unit and then triple-click on the elites is quite tedious, especially when it's several buildings. I guess I could work with units being part of two control groups, does any mod provide that?