...My head hurts...
BUT I HAVE AN IDEA I WOULD LIKE TO SHARE!!

infantry- non meahc pilots
Whatever we call our ground meahc forces 
Maybe air support mecha
Ground support mecha, like tanky, large, bulky mecha
Siege Meahcs- long distance cannon equipped mecha or sniper mecha.

Then have a squad of 6 to 8
Engineer 
Captain
2 or 3 Ground Mecha Pilots
2 Ground Support Pilots
2 Air Support Pilots
Infantry
 
Mabye something like this will work, but maybe in a larger scale, you can have multiple squads or just one, were the whole rp will focus on one large squad were we all will choose on role.

:P