Yes, a soft sided carrier is allowed and probably your best choice. Sherpa brand is most popular.

Do call AA and let them know you're bringing the dog. They will reserve your space. (FYI - This will also prevent you from doing online checkin). When you arrive at the ticket counter, they will collect the fee. They may also check that the dog fits in the carrier. At 15 pounds, it may be a bit snug, but with my Westie, I've always been able to convince them it's ok.

Sicne you are travelling to Canada, there may be extra regulations. I'd look into those also.

At some airports, there are signs that TSA will need to see the dog's travel receipt, but I've never actually been asked. At security, you'll need to take the dog out of the carrier. The carrier gets xrayed, while you walk or carry the dog thru the detector. (Avoid using metal/chain leashes, I suppose).

I'm not sure the underseat space is any bigger in F on the 757/MD80s... wider maybe, but not higher, and there's electrical stuff that cuts into the space also.

Flyertalk has a brand-spanking-new Travel With Pets forum that you should visit.
http://www.flyertalk.com/forum/forumdisplay.php?f=645