Green Ronin's take on Gehenna.

Also, it looks like I have some old notes on encorporating the Gehennan lords from that book.

Using the same rules, the other Yugoloths would be "exarchs" of the following:
--General of Gehenna- war, intrigue/secrecy, innovation, the yugoloth race
--Oinoloth- despair, pain, pestilence, tradition

Most of the fluff of course has to be ignored-- aside from their mercenary behavior, the Daemons in The Book of Fiends behave nothing like they do in D&D, and of course the Gehenna in the book is nothing like the Gehenna in D&D.
Nonetheless, some of the materials are useful-- with some makeovers. Many of the monsters and most of the "exarchs" themselves are quite salvagable. The exarchs simply need to be tied more with the intrigue-related aspects of the sin they represent, and instead of tempting mortals, they primarily tempt other fiends (and other non-fiend outsiders)

--Wrath-- Tyrexxus-- change to Retaliation and guerilla warfare; he belongs on Carceri. Should be less direct than depicted in the book.
--Lust-- In'nassi-- change to Seduction, Temptation, and Torture; it belongs on Gehenna. The seduction and temptation portfolios apply to the use of seduction in subterfuge and politics, rather than pure primal lust.
--Sloth- Viasta-- don't change. Belongs on Gray Waste.
--Gluttony- Yungo-- add consumption to the portfolio. Belongs on the Gray Waste.
--Greed- Mytaxx- don't change. Belongs on Gehenna.
--Pride- Gravicarius- don't change. Belongs on Gehenna.
--Envy- Ulasta- don't change. Belongs on Gray Waste. Probably a half-fiendish dragon of Yugoloth descent instead of a full-blooded dragon.

That would bring us back to how the "Altraloth" exarchs would work. Here are their portfolios:
--Bubonix- (Carceri) Disinformation, plague, war secrets, battle confusion, ambition
--Cholerix- (Carceri) bribery, mercenaries, haggling
--Chronus/Cerlic (River Styx) the River Styx, travel
--Taba (Wanders the Lower Planes) espionage, hellbreakers and abyssbreakers (hellbreakers are a PRC in Fiendish Codex II who specialize in breaking into an Archdevil's stronghold)
--Xengahra (Outlands) despair, outcasts, betrayal

Would that actually work though? Exarchs of the Hordlings? I was always under the understanding that they were the Gray Waste's answer to imps and mephits.

The role of hordlings is poorly defined in D&D. They're just gibbering, somewhat chaotic "hordes" that roam the Waste, and they aren't given a major role in planar politics.

But really, they're the neutral evil equivalents of tanar'ri and ancient baatorians in the sense that they evolve directly from larvae - and thus mortal souls - without needing any outside force to intervene. The yugoloths, who have no direct connection to souls, are their own thing, an artificial race that sustains itself mostly via their three towers. It seems that at some time in the distant past, the 'loths must have imprisoned or enslaved their leaders and left them leaderless and rudderless, unable to mount a significant challenge to yugoloth domination of the Gray Waste.

I wouldn't compare hordlings to imps and quasits, who have to be created deliberately by tanar'ri and baatezu superiors (not to mention they can grow a lot bigger).

I think you're right that some of the daemons in The Book of Fiends could work as extra yugoloth castes, if you wanted them. All of the ones labeled "mercenaries," for example.

I'd divide the various fiendish races into multiple categories:

Born from the essence of their plane
Yugoloths

Born from primordial larvae
Obyriths, hordlings, ancient baatorians

Born from petitioners
Tanar'ri, baatezu (indirectly), hordlings

Sexual reproduction
Night hags, barghests, diakka, nightmares, etc.

These categories aren't hard and fast ones. Elder baatorians (in the form of nupperibos) can now be born from petitioners. Both tanar'ri and baatezu can now form directly from the essence of their respective planes. Tanar'ri and yugoloths can reproduce sexually. But I think the categories are useful, anyway.

I think when the General of Gehenna first cast Law and Chaos from the yugoloth race in the form of larvae, there were actually three groups of larvae, not two. A group of neutral larvae also formed, and became the first hordlings.

I think instead I'd keep them as hordlings, but make them imprisoned somewhere.

Well, if you want! Maybe "daemon" is a generic term for all natives of the Gray Waste, like "demon" and "devil" comprise multiple species.

My personal opinion is that hordlings are the neutral evil petitioner-spawned fiendish race, making them roughly equivalent to baatezu and tanar'ri, both of whom make more of their kinds from petitioners one way or another. But probably the Green Ronin version of daemons should be used however the individual DM thinks is most appropriate.

Oh, man, that has even more insidious implications since hordlings are NE-- I think the idea of losing their chaotic nature might be even more frightening/disgusting than becoming the quasits of the Grey Waste.