Legally speaking on the second question, I'm pretty sure you'd be required to buy the family pack. That said, Apple has thus far been wonderfully nice about copy protection--meaning there is NONE on the home OS (Server has a serial #)--so unless things change significantly with Snow Leopard you will probably be able to.
As for the first question, I don't think anybody knows for certain yet. Apple has traditionally used the "upgrade only" OS install discs as CPU drop-ins, meaning they get tossed into the box of Macs preinstalled with the earlier OS--this lets Apple (and other retailers) clear stock of old models with nothing but an extra DVD tossed in the box. Apple has never sold these upgrade only discs separately to this point, however--anything you bought on its own was a full install (even, I'm pretty sure, the up-to-date discs).
I would say, though, that it's VERY likely they'll be full install discs. Certainly, they'd have to at least accept 10.4 installs, since someone could be buying Snow Leopard for an early 10.4 Mac that they'd never upgraded to 10.5, and I don't think Apple wants to bother making that small group of people buy a copy of 10.5 as well as a 10.6 upgrade. Given the relatively narrow range of machines it can be installed on I don't see Apple bothering with any sort of check.
