Don't confuse controlling a unit with enslaving it. The only way to enslave a unit in FfH2 is through the Slavery civic (yes, slaves can be awarded or purchased without Slavery, but those slaves are not representative of any specific previously existing unit). Units that are enslaved have essentially surrendered before being killed in combat, and had their weapons and equipment taken away. This is important because it leads one to understand that races that would die before surrendering (undead, demons, angels) or that are so dangerous that they could slaughter armies while naked and unarmed (avatars) would never end up as slaves.
I am not confusing anything... you are just not following me were I want you to go

one could suppose that :
1) slaves units are composed of few guys : 1 represented team, so maybe 10-50 guys;
2) combat units with 3represented units (or 5) are combat companies : normaly 50-200+ mens. (or 200-500) (you don't defend a whole city of 10000 inhabitants with only 3 LB (9guys with longbow) ==> hence each figure represents a team of longbowmen.. hence I demonstrate my assuption.
-->it doesn't causes any problem to have 10-50 guys taken into slavery while the other guys are commanded to follow you... just say that the slaves one were camp followers or disrespectful/hardheaded guys that displeased your commander.
3) combat units with 1 guy represented ... much more difficult : either a little patrol size unit, either a hero... I don't think a hero should be able to become a groupe of 10-50 slaves... I don't think either that an shadow teams should be enslaved, neither units with loyalty. But archmage, mage, adept, High priest, priest, acolyte... fawn... all other 1man units can in reallity represent a small team of men/creatures.. and thus be enslaved.
4)Now dragons... maybe when upon dying, sometimes, the dragon incarnates in a young man (kurios).. with slavery one can maybe enslave this guy.
then again, with subdue beast or command promo, upon death you may subdue/convert the dragon, but maybe he already has incarnated part of himself into a boy...that you can enslave... (When the kurios build the eurabates, they don't lose their leader.... the boy still exists)
5) the undead-demons-angels : can you command them ?? I think not.. not in the FFH I play.
Should you be able : ? no ! those are not mortal ones... they are dédicated to their master and will forever refuse to follow you in an army.
should one be able to enslave them ? why not ? mayb you can coerce them into doing small work ... and they may obey, waiting to outlive you and escape... as for undead, maybe with magic you can make them obey you a bit... but not enough to fight.
in FFH, one can imagine Shaeim following Order ... one just has to work his mind to find an explanation... If you can do that.. why can't you imagine scenariis like those I told... or other one that allows you to explain a slave of the dragon race or make you want to be able to have an angel slave !!!