This is great discussion. A pity I'm pretty tired with the release and need some rest to figure this out properly.
So here's my two cents, not conclusions, just rant when I'm not quite clear in the head:
Yeah, small legion has some psychological benefit too, I didn't know this well. There's a thing though, isn't the core troop promotions there to represent the prowess of Guan Yu himself, and the whole legion promotions there to represent his commanding ability? So, if core troop promotions and legion promotions are done properly, we should have the benefit of both world, in one hand is the hero's god-like warrior might, on the other hand is the hero as a commander in chief. After all, Cao Cao and Zhuge Liang are heroes too, they won't fight and charge, so they don't get some of the core promotions, but they get good legion promotions to portray them as great commanders.
There's still another potential conflict I'd like to point out: right now the unit combat level decides both the power of your legion and the size of your legion. It works fine if we understand the power benefit of combat level promotions as coming from the hero's commanding ability. It works not so fine if we think of it as the hero's own warrior might. It seems players more likely to think as latter.
About giving player choice whether to form pure combat type legion or combined legion, I love this idea. How could we achieve it? If pure combat type, your unit combat promotions are the one according to your hero's unit combat level. If 2 combat types, then each combat type suffers -1 penalty as to combat level promotions. If 3, suffers -2. This is a way to exchange legion size with unit prowess.
The commander range: there's already commander range in now, 2 plots, more than that your legion member automatically quit. A promotion to extend that size sounds cool to me. May be a little underpowered though.
I would try making some prototype with these ideas and post as beta release after I finish the much needed patch A.
