I think making each of the leaders of the magic colleges a pretender choice really limits what the Empire is and can be in the game. You could do exactly what you are proposing by simply summoning the one master of the magic college that you want and just working within their magic paths and ignoring the others.
I would rather have them all as choices to summon. Who knows you might just need those other college heads later on to give the Empire a splash of a certain color here or there