This is a good thread and brings up a long debated topic: Full-range in the rear or not? My opinion is that if you can do it (meaning, afford it), then full-range in ALL channels is the best solution and the most enjoyable. One of the problems that can occur, if running the rear as "small" and using a sub to output LF, is the issue of LF directionality. Meaning, if you have let's say restricted LF rear speakers, and you set them up to crossover at 80Hz (kind of the standard), then it is VERY likely that LF notes intended to be perceived as occuring in the rear will in fact seem as though they are coming from the front. Yes, I know LF is supposed to be non-directional (depending on frequency of course), but what most folks seem to forget is that this is in an ideal world. In the real world, you will have certain objects vibrate in your room at given resonance frequencies which even though not coming directly from the sub itself (shouldn't be a problem in a well designed sub), will cause serious directionality issues. This is in addition to notes above ~25Hz that can be localized in and of themselves. So, what the heck am I saying? I don't know, I forgot ;-) Seriously, if you can run full-range all around do it. At the least, run two subs, one in the rear and one in the front.
Marty...The Ospreys would be overkill for rears unless you really have the extra money lying around. The Swifts TRULY do go down to 35Hz, so you wouldn't get the lowest notes in the rear, but it should be sufficient. In addition, the Swifts are front-ported, so you can put them right up against a wall (disclaimer: I am a dealer for Meadowlark).
Best Regards...Mike - Father & Son Audio