As far as I know, the 23-inch tire "rule" is not set in stone. It's a generalization--mostly based on Sears/Craftsman standards--which are derived from their forebearers...Wheel Horse, John Deere, Cub Cadet, etc.
With that being said, I think you'd be hard pressed to find a tractor with 20-inch or smaller rear tires with a sleeve or 3-pt hitch.
With regard to mowing speed. Most people mow too fast with their riders. I see it all the time. Guys on those new Zero-turn mowers are flying when they mow. For them, time is money I suppose...
Heck, I wait all week to get some quality "seat time". I want to make it last
. Don't get me wrong, I'll speed it up if I'm in a hurry or if I see bad weather rolling in, but slow and steady wins the race
