There is a trick to know what are the max elements a unit can contain that is useful to see what's missing on a unit.
First select the unit you want to know about on the unit panel. Then control+left click on another unit on the same stack to select both.
And finally put the mouse over the + special order button (the one used to merge leaders with units)
A tooltip will tell you that you cannot merge the two previously selected units and detail the max number and type of elements they can contain.
Regarding your second cuestion, no you can't modify whats on a unit in game.
But if you get into modding i bet you can do that and much