Re: 20 Inch Rear wheel on a 26 inch bike?
Very true on the cranks... you need the basic ground clearance but a little more to prevent road contact if pedaling in a turn. The 20" wheel will lower the back of the bike 3" but if the cranks are about in the middle of the wheelbase they may only drop about 1 1/2" or so, but the further back from the middle of the wheelbase the more they will drop with the smaller rear wheel. A set of short cranks can be taken from a 20" bike to give more ground clearance but these will also offer less leverage and be slightly more awkward to pedal if you're used to the regular long cranks.
You can measure your ground clearance you got now then subtract 1 1/2 or 2" from that to get an idea where they will be after the swap.