the 20" rear wheel will.work just fine and it will still handle fine, a 20" on the front is where the handling issues begin because it git very touch and will cut side to side quickly.
pedals off a 20 or 24" bike should give you all the ground clearan e needed and you can use a 28T sprocket on the rear wheel to keep a decent gear ratio, a 40+T on the rear and it will be a low speed hill limber that might even go up a wall.....lol!
to know what you have for sure its very easy to put the 20 wheel on the i tended crame and heck all the points of concern out.
Ive had 20" wheels on the larger frames andthey a tually handle better than on the 20" frames, the straight line balance is more stable because of the longer wheel base, when you shorten the wheel base is when they get more sensitive and will respond quicker to the bars being turned a little.
Ipersonally do like a 24" wheel on a 26 bike for the rear or even on the front also, lower center of gravity and the bike handles better.
give it a whirl, you may like it if you can get the gearing like you want.