Here's a list of the tribes and the info I've gotten so far:
Kothri ? Probably a shortened version of Kothringi, who were a race of men mentioned in only a few in-game books hailing from Black Marsh. They were said to look like men only with metallic skin and were quite common in Black Marsh during the Second Era. They were eventually wiped out by the Knahaten Flu in 2E 560. Their origin is unknown. Some say they are the result of Nedes and Argonians interbreeding, others say that they were simply Nedic settlers that adapted to life in Black Marsh. From what the Adabal-a says, some must have lived in the Niben area if they were enslaved by the Ayleids. Other non-argonian peoples who lived in the Black Marsh besides the Kothringi were the Orma, Yerpest, and Howallli men, and Barsaebic Ayleids and Cantemiric Velothi mer.
Nede - http://www.uesp.net/wiki/Lore:Nede
Al-Gemha - ?
Men-of-'Kreath ? Not from the Niben. Apparantly 'imported from the North,' so they were probably from the city of Falcreath in Skyrim. Also mentioned in The Song of Pelinal.
Keptu ? Nothing known of them except that the name Zuathas is of Keptu origin, and was given to a Nede known as Zuathas the Clever-Cutting Man.
Men-of-Ge ? Destroyed when the Flower King Nilichi made a great sacrifice to an insect god whose name is now lost. One known man from this tribe was called Garid, who once saw Pelinal Whitestrake go into his infamous madness in battle. He avoided Pelinal and waited until the madness had left him before joining him for a drink and asking what the madness felt like, to which Pelinal replied "Like when the dream no longer needs its dreamer."
Al-Hared ? ?
Men-of-Ket - ?
I'm really curious about these peoples and their enslavement under the Ayleids.
If anyone has anymore information on them, and on the Ayleids themselves, please tell me.