I know nothing about Fred Cain cutters, but I operated a 5 foot (625#) Land Pride cutter behind a smaller Deere 670 (16.5 PTO HP, 19 HP Gross). An internet search on Fred Cain cutters revealed the 5' unit weighs ~ 700 pounds...can your 3PH handle that? You'll want at least 200# of suitcase weights up front if you don't have a loader.
As long as I had the loader installed, there was no problem moving the cutter raised (watch those turns though...slow and easy is the way to go).
As for cutting, I was going through thick brush, most of which was as tall as the cowl of the tractor.
So, with common sense (and weight on the front of the tractor), you can operate a 5' cutter with a 20 HP tractor.
By the way...what kind of tractor do you own?