OK, for all those (like me) who don't have time or patience to play multiple campaigns with same factions in order to see all COW units I've modified attached file so you can, after extraction to COW data folder, do the following (only for version 1.4.2 & 1.4.4 compatible):
1. Recruit all special units that belong to that faction (with all factions) like any other (when you meet building requirements).
2. Recruit all mercenaries that belong to that faction, (empire can't hire chaos dwarves for example) with most empire and all other non empire factions (when you meet building requirements).
3. Build chapters and recruit all empire knights (with all empire factions)
4. If you like rats, you will probably like this file, as a special treat, I implemented five skaven units that creators of this wonderfull mod left out. You should be able to hire them with most factions (soon with all) that can build mercenary camps.
5. This latest file unifies all previous files, and further more, now you can recruit heavy free company and cursed company.
6. Second file (alternate) gives you almost all the same things (+two more mercs for all empire factions); but the main difference is that in this file construction of buildings is somewhat shorter. If you play short campaign, you rarely can see all that game has too offer because town development takes too long. So for those who can't wait 12 turns to build that huge cathedral - this is a file for you.
P.S. Save your original files; will also upload clean file (if I manage to find it).
You can't recruit all mercenaries with every empire faction, because, when I enabled it it was to much for game to handle and had constant crashes.
If someone wants all mercenaries with some other empire faction, you can further modify this file (but not too much). If you don't know how, post me and I'll do it when I have time.