Health doesn't apply differently to SP or RP, to to any positions for that matter. The lower the health, the more likely that the player will get injured.
As to durability . . . you really should be looking at both durability and stamina together for pitchers.
In general:
DUR > 20 and STA > 70 = SP
DUR > 30 and STA < 65 = LR
DUR > 40 and STA < 50 = RP
Any oddball combination other than these will most likely be either LR or RP. In general, for RP, DUR > STA.
Also, it's always OK to use any pitcher as an RP . . . but if he has STA > DUR, he will fatigue and recover a little differently than a more traditional RP.
You really wouldn't want to use a guy with RP type DUR/STA as an SP because he won't be able to pitch very deep into games.