I know only some basic stuff about ohm load, speaker impedance, series and parallel connections, but in my opinion, I still feel that they are wrong on this.

If they allow the 8" 8ohm, they should allow you to use the 12" 8ohm. The watt capacity of the speakers should not be an issue here when the speaker impedances are the same.

If you were planning to connect both monitor speakers, than they have a valid point.

Well, its a bummer but they own the goods and they have the right to set the rules.

Good luck. Hope you resolve the issue, find and alternative, or another supplier.