He's making some generally good points with some screwy specifics, so it may be that you're not following his points, or it may be his knowledge is dated and hasn't carried forward to today's products. His Bose opinion leads me to believe the latter.
Subs do not have to be matched to mains. The frequencies a sub provides do not affect the harmonics of the mains, so a sub from any manufacturer will be compatible. However, his pointing out a possible gap in frequency coverage makes sense; but again, in your case I don't believe that to be true, Monitor 30s look like that have reasonable flot response down to 63hz, the Bic covers 25-200Hz, so there's the overlap. Unlike mains, subwoofer impedance means nothing to the consumer, as it has it's own power supply and doesn't have to be matched to a suitable amp. I'm guessing he meant that the impenance of the driver would affect how loudly it plays, but that's not the primary quality measure. A larger drive can move air more efficiently, so all else being equal it'll be better. Of course there are other design considerations, so just going big doesn;t work either, but Bic has a good name. Now if you're placing it in a small space, you may not need the speaker size and can use the money to purchase better design, or parts elsewhere. (Meaning if you don't need 12" of air pushing capability, you can move up a brand and get a smaller Hsu or SVS, or some equivalent manufacturer. Botttom line, I think you're on the right track.
Going back to your uncle, his thought on rating a system as a whole is valid, but I don't think he's using the correct criteria to make that assessment.