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.
Tree: Screenshot by Lightshot
Buff-Bar showing the distribution: Screenshot by Lightshot
I guess you have noticed but let me point out this mechanic can also be abused to keep a Skeleton Archmage/ Arch Deathknight/etc out after speccing out of it and recasting 2-5 normal-size Skeleton mages/deathknights/etc. Giving you basically double the intended amount of Mages