[Sauron the White]

from Alatar

But I thought that Gimli admits later - at least in the Books - that he wouldn't do well fighting from horseback, and yet he places himself above his enemies (and behind his companions), which, to me, would be less than advantageous.

If you look at the height that Gimli was working from on Balins tomb, the swing of his axe would have been perfect for an orcs head. I believe it would be a good foot or two shorter than if he were mounted upon the back of a decent sized horse. The foe who has to swing up at a defender usually is at the disadvantage. Gimli probably knows that as a hardened warrior.

I also think that he was making something of a statement with his stance and body language which is mirrored by his words 'let them know that there is one Dwarf left....etc" and sorry but that is a paraphrase and not a quote. He wanted to be the center of attraction for the invading Moria orcs and that was one way to do it distinguishing himself from his companions who were on the ground.