The transborder stuff is fine. AA is pretty much free to operate into any Canadian airport, provided there is a CCRA and CIC presence (Customs and Immigration, respectively).

Where AA will need codeshare access is on the Pacific route network (where they are badly represented), and domestically within Canada (where they are entirely unrepresented).

I have a feeling, they will try and develop more transit through YUL, YYC and YVR, leaving off YYZ because of the twin problems of poor physical connection, and more restricted slot access.