Re: 20 Inch Rear wheel on a 26 inch bike?
Usually a very bad idea. Makes pedal clearance a nightmare and also makes handling very twitchy due to trail changes sometimes. (Depends what fork you use)
Better off to mod a 20" frame than to stick 20's on a 26" frame in my opinion.
If more is better, then too much is just enough.
If you can't afford it, build it yourself.