Known Knight-class Units and Their Sides
Knights appear to be advanced (elite) versions of other troops. Stanley the Tool's Knights were drawn from all the (Hobgobwin) regiments in his (former) empire. Transylvito's Skanks are just a special class of Guard Goyles. It is possible that all Knights are promoted from lower classes, although it may be possible to Pop them as Knights.
Perhaps Knights are always able to ride Mounts (thus the name Knight). If so, they must always be advanced forms of non-Heavy Units, and must not become Heavy Units when they become Knights. If so, then there would be no such thing as a Twoll Knight, for instance. <- Infantry, by definition, fights on foot. I doubt that "knight"-hood has anything to do with mount, however we might associate knights with riding horses and jousting.
Real World References
The word 'knight' in English and its Japanese cognate 'samurai' both derive from words meaning 'retainer' or 'servant', which places the knight alongside older traditions of cauldron-kin and hearthbands as a fighting man in direct service to a ruler. Thus it would seem sensible for the knight class unit in Erfwordl to be part of a hand picked elite directly responsible to a faction leader.