I was barely able to answer this question because I don't think any of them would have a successful career and the one I did answer (Roy Jones)- I haven't really heard much about him getting into MMA.
All these fighters are old and extreamly past their prime. I would say Miguel Cotto or even Manny Pacquia would have successful MMA careers if they just focused on NOTHING but BJJ and Wrestling for a year or two...
But to be MORE realistic... What about young upcoming Boxers like Timothy Bradley and Julio Cecar Chavez Jr.?
Now they should make the transition!!!!
-Anyway I picked Mayorga because he's the youngest by a longshot at 36. Roy Jones is 41, James Toney is 41, Ray Mercer is 48..
Rahman is 37 but I don't think I ever seen him fight.