Githyanki ranks

They do sometimes use mercenaries, because githyanki healers are comparatively weak compared to full-blown clerical skills. However, ghustils are quite useful and would generally be considered more trustworthy.

In 2E, ghustil are described as a form of hr'a'cknir, one who is able to focus astral energies into a limited selection of spell-like abilities. One caveat to this is that the documentation also states that hr'a'cknir powers are only usable on the Astral Plane, which makes them useless on other planes. (See "The Guide to the Astral Plane")

In 3E, ghustil are slightly different. While I don't see any mention of them in the core materials, they are described in detail in the Polyhedron section of Dungeon 100. They are described there as githyanki-only prestige class, more adept at healing than bards and rangers, but not as adept as clerics. Rather than having spell-like abilities, they actually get spells. It says they generally start out as wizards, sorcereres, bards or psions, and then move into the prestige class. Prereqs are: heal (4), arcana knowldge (, plane knowledge (, skill focus (Heal), githyanki race and ability to case 2nd lvl arcane spells. Again they can only select certain spells, but the list is a larger. Unlike 2E, 3E ghustils can "focus astral energies" in any plane connected to the Astral Plane -- which in the D&D cosmology basically means that their powers work anywhere. (For detailed information, please see Dungeon 100.)

Hope this helps!

I believe that a sergeant in the githyanki is called a sarth. There is some info on this in the polyhedron section of Dungeon #100.