Does anyone know what kind of damage can be done if the wrong transmission fluid was used?

depends on what atf, and for how long.

Atf Type F for about 7,000 miles.

Not sure about the damage... you'll probably just have to see if any problems develop later on. I would think the worst case would be damaged friction discs, requiring a rebuild. I kind of doubt it though. For a drain/refill you only get out around half of the fluid, so you should have a mix of fluids in there. If you want to completely remove the type-f, it may require changing the fluid several times in succession. It will be kinda pricey.

Type-f fluid's main difference is that it has a "grabbing" agent in it, to promote better clutch engagement. I used to use it in General Motors transmissions (they call for Dexron fluid)I rebuilt for my hot rods (when I was a kid). I think as long as you flush it all out and put the right stuff back in, you should be ok. I'm not sure if the X's have a drain plug in the converter, but if it does I'd drain it too. The whole system should use around 11-12 quarts. If all you did was top it off with Type-f fluid, I'd say just let it ride, you should be ok.