You will have a hard time finding a parts store that can pick and chose. :( The goal for the automotive aftermarket is minimize inventory and increase profit. Even brake parts are too often 'close enough'.
OK. I'll get off the soap box and give you some links so you can do some checks.
One check is figure out what is wrong with the alternator that was on the car.
Two figure out if its 'safe' to run the replacement.
This is easy. Do a current draw test. If the rotor is drawing a lot more current than the shop manual specifies, there's a good chance the regulator will fail sooner. More current through those small wires is more heat.
A revised squareback was designed from the begining with rotrs that draw more current.
However a roundback or regular squareback that has been 'rebuilt' could have a rotor that also draws more than stock. So its worth checking.