Silva is clearly one starter. I hate Mays as a starter with the 16 DUR/68 ST combo-- I'd put him on a 50/55 pitch count in a 5-man rotation, which will get you only about 3 2/3 IP per start-- but Morris has a huge platoon differential, and because the sim isn't very good at pinch hitting to gain the platoon advantage, those guys do better as relievers. Your call.
You're correct in assessing these guys as legitimate back-of-the-rotation or mopup guys; used correctly they'll all run ERAs just a touch above league average.