As a 121 pilot, you can "commute" on any airline that has a jumpseat agreement with your company.

The issue that comes into play is what happens when you are unable to commute due to full flights. Some companies have "commuter clauses" that cover your absence, provided that you meet certain requirements.

I don't know what PCL's policy is towards missed assignments due to commuting, but the typical clause is that you must be listed on and attempt to make at least two company flights that put you in base by your show time. For some "express" carriers, you can include the mainline flights as a "company" flight.