[Endoperez]

Liga's Manual Addenda mentions that high strength and flying add to the sieging ability, so atleast siege is more complicated than 1/unit. This hints to castle defence being more complicated as well, but I only know that being mindless makes you poor at it.

About summons being commanders: units/commanders are not differentiated besides the way they are added to the game. You can #addrec a Vanadrott, and you have a very expensive Van that gets some nifty magics if Gift of Reasoned; you can also #addcom a Satyr Hoplite or what have you. We cannot mod change whether creature spell summons is a commander or a unit.
Your best bet is probably to use a rarely used spell. One example is Corpse Man Construction. If you mod that, remember that Lightning Rod and Staff of Storms increase its effectiveness.

Humans should be about 32 pixels high, and Modding Manual mentions few other heights as well. However, it is best to start from a right-sized picture of a unit that already is of the same general type, and add on that. I'm not sure if Gandalf's pictures are of the right size, but they seem to be.