I have a pitcher who is currently 56 stamina and 26 durability coming up through the system.  What would be the best way of getting him the most innings?  I've debated between starting him with a 4 man roatation and lower pitch count, 5 man rotation slightly higher pitch count, or just making him my only long relief guy so he'll frequently come in mid game.  Is there a benefit to any?

I'd start him as often as you could.
Starting is always better imo, because YOU choose when he starts rather than hoping sparky gives him the ball from the pen.

Doesn't matter what size rotation, just move him up if/when he's ready ahead of time.