I think I still have my Fimir models up in my attic, from my Fantasy Battles.
Meargh, witch hag, only females, sterile
the Dirach, daemon fiend, daemon summoners
Warriors, and the elites = breeders
Shearl - Fimir Slaves = weak fimir
Human slaves - woman are used for breeding, rest mostly used for sacrifices to the demons
Fen Beasts - not sure if these are the same as the bog beasts as they are magically created from the souls od the dead
Boglars - Allies - sub-species of gnoblar that live in marshes
You need to think of them from the point of view of villagers, Think the film The Fog.
A mist rolls into the village, there are screams in the night, the panic, cant see where your going. When the sun rises, the survivors speak of demons of the mists, one eyed beasts. Armies that enter the mists or bogs are never seen again. Once they are removed from the cover of darkness and mist, they are easily dispatched.
The whole Fimir history in Warhammer is so very confusing, even the books seems to have different views on them.
From what I been reading, the Fimir were either created in Albion during the destruction of the chaos gates, or if you read the Albion book (page 20) it also mentions them being the first born race. It a very confusing read. It also mentions a Horned God, the father of the Fir Bolg, as they call the Fimir. It also calls them Lochlann and Fomorians. The whole book seems to be an attempt to convert Irish folktales into Warhammer history. Also White Dwarf 102 has some info on Fimir and an wfrp adventure called "Theres a One-eyed fellow hiding to the south of Kammendun". (Wierdest adventure name I ever seen so far).
Below is an idea I had while researching the Fimir, since the Albion book came out.
Since there is so little information on the Fimir, maybe after the destruction of the chaos gates, it took 2 years for the high elves and the truthsayers to contain the corruption via the magic circles, on the isle of Albion the storms of chaos created this Horned god who created the Fimir. But he didnt like what he created, and left. Then sometime in the future he created the skaven. So the Fimir are jealous of the Skaven. Only basing the Skaven part on your info on their hatred of the Skaven, I didnt read this in stuff I had read.
This is all based on stories though and two gods having the name Horned in their titles.
Before the Albion book, I just thought the Fimir were one of the Old Ones slave races giving the duty to use the mists to hide their research stations, who during the chaos storms got corrupted to sacrificing other races so they could survive. The stories mentioned an Isle of Mists they came from. Since back then we had the Slann of the temples, Zoats of the forests, Fimir of the bogs, Dragon-Ogres of the mountains. It sort of made sense.
Wasnt there a collection of demons in 1st edition that were just evil and not connected to the chaos gods? Imps etc. Wish I could find my 1st editon books, I know i sold the 2 books of chaos, but I am sure the summoning spells mentioned other demons for law/Good/evil etc