I think one of the later episodes (GROPOS, maybe) says that Garibaldi was in the ground forces and didn't see much action during the war. Can't really remember.

My fan rationalization for the Ikaran not being much smarter than a pile of stupid rocks is that the device was designed for the Ikaran brain, and it was having trouble adapting to a human brain. If it had been attached to an Ikaran it would have been very smart and deadly, not just zombie-with-a-gun.