Eatersmeter,
No need to take the Private stuff again. You can be issued with a restricted FAA Private Pilot Certificate on the basis of your Canadian CPL. The restricted Private Certificate meets the pre-requisite requirements of having a private pilot certificate before getting your commercial certificate.
You will , of course, have to take the written and flight test, and meet all of the aeronautical/flight training experience requirements and have a CFI endorse your logbook before you get a US Commercial Pilot Certificate. By the way, unless you also get a U.S. Instrument Rating, your Commcercial Pilot Cetificate will be limited to flying passengers for hire during the day and not beyond 50nm from departure point.
As far as the instrument rating goes, to get a full U.S. Instrument Rating, you would need to take the written, do the flight test and meet all of the appropriate requirements in order to get it. You could have one issued on the basis of a Canadian Instrument Rating by taking the Instrument Foreign Pilot written test. However, you would then have to maintain your Canadian Instrument Rating current in order to exercise IFR privileges in the U.S.
Ray