Produced by wonder/small wonder is a good idea. I like how you have it set up with Mystara. I think they should also have a high attack, low defense rating to show their effectiveness in secrecy but the penalty for being found out.
Also, since Kinboat's working on a Warder unit for us, it's got me thinking about how to implement them along with Aes Sedai. Here's what I've put together, from opinions of people in the thread as well as some of my own. Alerum does not want to have Aes Sedai as their own civilization, and he has made some good points to support his position. Wouldn't want the White Tower declaring war on rival nations, which is bound to happen no matter what. Even lowest aggression civs sometimes get it in their heads to become warmongers.
Solution = have them linked with a resource that has a high appearance/high disappearance rate (which makes it pop up all over the place, but for short whiles). This will display the rareness of the ability, and it'll make it difficult for one civ to make a whole army of Aes Sedai. Give the Aes Sedai the Enslave ability, so they enslave/bond their Warders. The Aes Sedai will have a VERY high defense, but an even lower offense rating (maybe even 1). This will prevent several things. (1) People (AI included) will not use them to declare war since they'd lose a valuable unit (demonstrates Aes Sedai vow not to attack unless attacked upon first). (2) Units are a lot less likely to enslave units when attacked upon (in defense), so you won't have Aes Sedai producing tons of Warders. Warders will be rare then, but I think that's how it should be since I view them as the second most powerful foot units, behind only the Asha'man.
Then Aes Sedai will be available to everyone. BUT as with Knights in the main game, they can be represented different within different civs. Shadowspawn get Black Ajah, Randland civs get Aes Sedai, Aiel get Wise Ones, Sea Folk get Wind Finders, etc. etc.
Whew! Think I got it all down. That's a lot to consider, so let me know any of your reactions.
And... Sorry for the long post.
EDIT: Oh yeah, they can also have transport foot unit abilities.