Well, is a question of find a way to add it... maybe the use is not going to be incredible high thanks to the scale but well, if you find a situation like this be ready is allways a good point.
Use + and - maybe is problematic as you say but nothing that Mr ( and Ms ) cant solve
And you allways can use letters...
I use yellow/gold to represent the situation present in PzB2 with Fireflies... main force is made of the weak tank (regular sherman) and only 1 is Firefly with portrait for the main tank in the section (i think use this when less than 50% of the vehicles)
Red represent a situation with a weakest tank in the section, for example you can have a mix of 3 Fireflies with 1 regular Sherman (in late war something like this was present in USA armored units... i remember read about sections with 3x76mm and a single 75mm because it can use smoke ammo and better HE shell).
If you dont see the symbol is a solo tank type section.
There are many options here, is an arty question and saddly here i am not the best to provide profesional examples (here you can see my copy paste mod to have Jison minefield icons and repaint defensive hexes with a letter)
Ummm you made me now doubt about the spot system but i find to that certain terrain is not offering enough cover to units movement, at least infantry, like in urban.
I dont think they are now thinking in spotting sytem... or not, is the top secret feature and we dont know what is it