I just specialised my Skeleton Mages to be Death Knights which SHOULD block Mages from being summoned as “Mages”. But no matter how often i re-cast the Summon Mage skill, i always re-summon one of two Death Knights while 1 Mage persists.
I picked the Death knight node while i had 3 Mages out already, this might have to do with it.
Buff-Bar showing the distribution: http://prntscr.com/r6ya6t
I hope this helps