The advice being given by any Scottish Power employee is bound to be wrong. They will tell you utter nonsense, and it changes each time I have phoned them. I totally refuse to deal with them on the phone now, they must write to me. That way, I have it all in writing. I suggest you do the same too.
I have two meters and I have managed to change tariff with Scottish Power. However what is apparently very difficult indeed is to change supplier, as none I've come across accept two MPAN numbers (2 meters).
Best of luck. I hope you have lots of patience. You'll need it dealing with this crowd.