For a "maiden voyage" DO, I would suggest a relatively short cruise - say five days or thereabouts, just to see how it goes. If it goes well (and I anticipate it would), further such DOs could be planned on longer cruises.

My preference would be for something out of a Florida port, but that's mainly because I live here and could drive to the port! Obviously other folks will have other preferences, I'm sure.

I'm not convinced that where the cruise goes is particularly important at this point since the basic objective is to meet and greet fellow FT travelers, right?

Either fall or spring would be fine - as you said, when the kids are in school and not on board - although I'm not convinced that FTers themselves are particularly "mature." Have you read Omni lately?