Heirarchy of the 'loths:
General of Gehenna
Oinoloth
Ultroloths
Arcanaloths
Nycaloths
Lesser Yugoloths
Above the Heirarchy
Baernaloths - no direct political authority, but their 'advice' is generally taken as gospel by the most powerful Ultroloths or unique yugoloths who either seek them out or who have such 'advice' given to them without asking. While they take no direct hand in the political structure of their children, they subtly twist and steer the direction of the overall goals of the yugoloth race.
Outside the Heirarchy
Altraloths (outside of the proper heirarchy, but raw power tends to elevate them in influence)
Order of Promotion:
Skerroloth (demoted mezzoloths) - Canoloths (demoted mezzoloths) - Mezzoloths < Dergholoths < Piscoloths < Hydroloths < Yagnaloths < Marraenoloths < Nycaloths < Arcanoloths < Ultroloths
* there are also Gacholoths and Echinoloths, but their exact place in the heirarchy has not been fully fleshed out. They're both lesser yugoloths, probably somewhere between pisco and yagna.
* Guardian Yugoloths are not included above because they're not actual yugoloths, but more something like semi-sentient constructs of raw soulstuff created by the yugoloths to serve as summoning fodder. If you try to summon a yugoloth without knowing the name of a specific yugoloth, odds are that you'll get the spell twisted and redirected to get a guardian yugoloth.
All lesser yugoloths can breed to produce mezzoloths, but most mezzoloths are simply 'birthed' directly from the raw essence of the Gray Waste, Gehenna, or more rarely Carceri. Among 'greater yugoloths', Nycaloths can breed to make nycaloths, arcanaloths can breed to make arcanaloths, and Ultroloths can birth arcanaloths by parthanogenesis. However this is frowned upon, and nycaloths or arcanaloths born into caste are largely second class fiends compared to those who rise up to rank from mezzoloth status.
Edit: I don't believe that I deviate from canon material on the 'loths, so much as I add a bunch of stuff in my SH. To that end, my treatment of the Baernaloths is speculatively extrapolated from the canon material on 'The Demented', but the histories, names, and lore therein in my SH is my own with the sole exception of Daru Ib Shamiq (who is a canon Baernaloth, but was never explicitely named as a member of The Demented).
If there are any specific questions about my SH treatment of the 'loths versus the canon material on them, I'll field those questions certainly, as well as generic 'loth lore too I suppose. Just had my first full day of work today at my job, so I'm happy and pleasant at the moment, I should spread it around I guess. *toothy grin*