To be honest, this is really only an issue for Mulch Shambler as far as I'm aware and isn't really important for other spells.
I personally think that Mulch Shambler should just pick a random shambler, since there probably isn't usually that much strategy in picking one over another (aside from placement for the AoE?) They are typically all in around the same area anyway, though they aren't so tightly grouped together as Erebus's Nightwalkers...