Hi, Poirot. It's been a really long time since you posted this so maybe not relevant anymore. Plus, I suspect ballroom is an entirely different ballgame than the Argentine Tango scene. I find that it's eclectic and though plenty of women do dress up, no one wears formal gowns. The main thing is definitely flexibility in movement, and you can accomplish this with any dress or skirt that's not confining (ie, a pencil skirt would be a bad idea, even with a slit). A lot of women wear pants as well--they are sometimes long, flared trousers, but often are cropped. And I see a lot of leggings or bicycle shorts-type of pieces under pretty tunics or short dresses.
I still have a very limited tango wardrobe, but I plan on growing it out this year. I bought two very nice dresses, with links below. And recently a third dress that was inexpensive and stretchy and I wear it with black jeans or leggings also. In the future, some pretty tops and skirts should do it for me, and a few more flirty dresses when they go on sale.
The green one is Tadashi Shoji from Nordstrom and it is my fantasy of how a tanguera would have looked in the 1940's. : )
http://youlookfab.com/welookfa.....-out-tango
http://youlookfab.com/welookfa.....o-part-two
This one is a Classiques Entier, also from Nordstrom:
http://youlookfab.com/welookfa.....s-ka-ching
Poirot, I also take photos at my teachers' monthly dances, so if you're curious about how women dress in an informal social tango context, have a look. (Ignore the belly dancer pic--she was giving a performance!)
https://www.flickr.com/photos/47758673@N03/