In terms of changing Heraldry based on rank what I would suggest is that the heraldry itself doesn't change, but the way it is displayed changes.
One possibility is to have the type of banner your army uses change depending on rank. The only rule of I am aware of historically is in Scotland, where the size of standard you were allowed to use was based on your rank. See http://en.wikipedia.org/wiki/Heraldic_standard for more.
The other bit about heraldry that changed with rank was the "achievement of arms", which are basically those fancy drawings of the coat of arms with animals on either side, a motto on the bottom and a crown on top. I'd go with something like Lord or knight gets a helmet on top, Baron gets a motto, Count gets supporters and Duke gets a mantle, but that's a lot of effort for something that is purely visual.
I agree with you on he other stuff lanceor and will get back to you on that.