I think you are saying that under Client Letter, you have several letters where the defaults are showing as Single Window Default, Double Window Default, ...

I think the only way to get what you want is to select your special clients from the client screen, when print the organizer for that set of clients using ONE of the special letters. You can make the letters two pages long (or more) to get more than one special message out. l

Short answer - there is no easy way to do what you want. I think it would be easier to print your special letters (maybe on different color paper) and use them as inserts in the organizer.

I also strongly advise that you wait until at least the end of November to start actually printing. Organizers are very buggy in the beginning. The same goes for Proforma - Maybe wait until next year when you are actually going to be filing returns.