Known Knight-class Units and Their Sides
Some 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 units of lower classes might be promoted to Knight. Archons do not seem to have a non-elite version and are known to be popped directly as knight-class units.
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. Normally Infantry, by definition, fights on foot, yet we saw some of Stanley's Hobgobwin Knights ride on Dwagons. So it is possible that Infantry are unable to use Mounts, while Knights and Warlords may do so.