EV11 - Different retention policy for specific message class

I want to archive IPM.Schedule.Meeting.* and REPORT.IPM*, but apply a specific retention policy to those files.

I find I am being left with folders full of these file types, because they are not included in my existing archive policy.

I have added the IPM.Schedule.Meeting.* class, and created a retention policy I want to use.

I then created a new Mailbox policy, which applied the settings I want, and selected the Message Classes I want.

I then created a provisioning group, and tied this all together, and applied it to one user.

It hasn't helped though. I assume a mailbox can only be assigned to one provisioning group.

Is there another way of acheiving what I am after?