Shayuri:
Complete arcane, warrior, adventurer, races of stone, races of the wild, PSCS (this is probably the best list of spells for a planar druid), MotP, planar handbook
WarShrike:
That works mechanicaly, though just annother point: Githzerai highly value freedom. It is very, very, very, unlikely that one would sign away thier freedom and soul after thier experiences with the ilthilids.
Kinem:
Again, a quasit with 12 class levels would have consumed enough greater Tanar'ri to be promoted as well.
You can play a greater fiend, but when you do so, you invite me to push you around a bit more. You are extremely old and have and have more enemies than a mortal could make in a lifetime if they tried. You are also acountable to your direct superiors in Sigil and indirect superiors in Cania.
Annother note: non-unique devils (except those that are authorized to travel to the Prime more freely like erinyes and falxugons) get more options through promotion, not through other sources (Devils aren't the most creative bunch). A trip to Sigil does not advance that goal; in fact it hurts it, because while you are away, your peers will prepare to move against you. Ditching Mephistophles will get you killed since other archdevils don't like to enlist gelugons because they are loyal to Mephistophles. You are more trouble than it's worth to protect. Same deal with the aminzu. Your options lie with either the Dark Eight (fighting in the Blood War) or with Mephistophles (pushing others around and serving as an oficer in his personal army).
Baisically, I'm not saying no outright, but given the niche that Gelugons fill, you'll need a far more compelling motivation to move to Sigil. He would already know a great deal about mortals (being tens of thousands of years old and dealing with them in past forms).
The most likely reason a gelugon would be in Sigil is because they screwed up big time and got asigned to command to command a smallish security detail in Hell's Gate, but this would leave little time for other things. If you want to play a devil, it is best to play one that deals with mortals (erinyes, falxugon, etc.) because these devils are suppoused to be in places like Sigil.