The inflatable wheels are now inflated. (But how air should they have?) The trick seems to have been to use a compressor. A sport-type hand pump just doesn't provide enough pressure to force air through the silicon tubing (which I have replaced with standard fuel tubing on both tires).
So this means I now have the inflated (but soft) 4 1/2" wheels and the rock hard 5" wheels, both of which fit on the gear. The larger 6" were, unfortunately, a complete waste of money.