> Actually you hit on one of the lesser-known rules of Camelot system, that +/-7 (aka perfect 5th)

Er, what? A perfect fifth is +/-1. That's why it's called the Circle of Fifths. +/-7 is a minor second.

9A and 2A only contain two notes in common - the other five will all clash. It's not really mixable in most cases without using special tricks.