This has probably been requested before, but it is annoying only being able to select 16 units, I know they can be saved as groups but it is still annoying. The best thing I think is when more than 16 units are selected their images in the top right corner stack so all of one units shares the same image and has a number on it showing how many of them are selected.
Also this would be pretty useful, when a units image in the top right is left clicked only that unit (or stack of units) is selected, also with less than 17 units selected double clicking the unit image would select all units of its type within the selection (just liking left clicking a stack of units). Right clicking or double right clicking should do the opposite removing the unit(s) from the current selection.
Thinking about it, it would probably be better to stack units straight away even with less than 16 selected. When more than 16 types of units are selected, they are all selected but the first 16 types are only displayed in the top right corner with maybe some image signifying that more than 16 types are selected or could get really advanced and add arrows to get to a new page of unit types selected.