I'd really like to train elves. I've been told I got to form an alliance with a non-human race, and their respective branch of foreign quarter buildings will be unlocked.
At the moment I am playing single player skirmish (I really like skirmish not campaign). Is it possible to unlock foreign quarters in skirmish? How do I form an alliance? I mean I got to click diplomacy button and propose an alliance to a cpu player? In skirmish I don't think there are elves or centaurs leaders. Or when I invade with my hero an elven province they propose me an alliance instead?
And of course the unlocked foreign quarter is unlocked only for that skirmish right? I mean if I start a new skirmish I have to form a new alliance right?
Thanks in advance :)

Yes, you enter a province and have an option to offer an alliance to the race living therein. Both in skirmish and campaign. This is not connected to diplomacy between players. Yep.

Thank you very very much ghaaroth :)

Maybe I'm stating the obvious,
buuuttt still:

You can the race populating the unconquered province and decide if you want to negotiate with them.

The Good:

Obnoxious Dwarves populate hills,
Stuck up Elves hug their trees in the forest provinces,
Ridiculous Halflings are getting fat in plain provinces. [edit] I saw those ugly little fellows in hill provinces as well.

The Bad:
Stupid Orcs live on hills,
Stinky Golbin populate plains, or maybe swamps.

The Ugly ones [edit] They are neutral and it seems you can form an Alliance with them:
Ignorant Centaurs graze in plain provinces as well.
Sticky Lizardmen rot in their swamps.

Lizardmen and Centaurs are Neutral races, so making an alliance with them is possible for both very evil and very good characters. As far as I know.

yes it is possible to form an alliance with centaurs even if you are very good (like i am right now)
but for an alliance you always have to complete a task (finding the axe of the dwarf king for the dwarves - killing 5 elves for goblins or killing 5 ogres for the centaurs - that are the ones i remember)
then you can build the respective building in the foreign quarters

btw. centaurs are awesome - rank 2 and better than horseman archers - and cheaper

oh yeah ofc you can't conquer the province you want to form the alliance with until you complete the alliance quest - and if another player conquers it the quest for the alliance is void.

Ok I just wanted to say I just allied myself with elves. I must say they are awesome! Elves archers are brilliant, in my modest opinion by far the strongest rank 1 unit (they have a high upkeep compared to other rank 1 units but they deserve it!).
Moreover an alliance with elves gives you the opportunity to construct a building that gives you +1 population mood in all provinces!!
I think I was lucky my first hero was a ranger. I believe you got to spend 3 points in diplomacy in order to get the quest that gives you the opportunity to forge an alliance with elves. First time I get there the dialogue with elves ended up at the first screen. Once I get diplomacy 1 I could bring on the discussion of another step. I had the quest only when I reached diplomacy 3 (I had to conquest a mage's tower to free one of their friends if I remember well).
Anyway I am really enjoying them, my heroes are killing machines with those elves as rank 1 units!

No you don't need diplomacy for the Elven alliance if you are good-aligned enough. However, if your karma is too low then yes - diplomacy matters.

Edit: And yes the elves are awesome - both as units and the +1 mood in all provinces!

I am playing skirmish, and I am taking only good karma decisions since the beginning of the match, but I was stuck at the first screen with all heroes, when trying to forge an alliance with elves. Maybe you need a very good karma to ally with elves without the need to use diplomacy skills.
During my current match I noticed that with 0 diplomacy I was stuck at the first screen (I had only the option to attack or to retreat). Diplomacy one I had the possibility to go further of another step. I noticed it was only when I took diplomacy 3 that I had the possibility to take the quest. Every skill in diplomacy unlocked me a dialogue option and allowed me to go one step forward. So maybe like you say you have 2 ways: good-aligned or diplomacy skills. I might be wrong since I have a very limited experience, but during a skirmish match it takes a lot to have a good karma? I mean I was at level 10 with my hero and despite I took only good decisions I had to take the 3rd diplomacy skill to perform the alliance, because the other heroes were still stuck at the first screen.