With specialists, you might have to worry about appearance fatigue. I wouldn't use one if I had 6 or fewer relievers, as a specialist would probably create an extra pitching change for every game he plays.
A specialist will have a 5% advantage above neutral, and a 10% advantage above the opposite hand in effectiveness, so they are worth using if it makes sense for your team. Season stats can be misleading because of switch hitters. A switch hitter will always be listed as a RHer in your LH specialists totals. So, if your specialist faces 50 LHers and 50 RHers, that means he actually faced something like 50 LHers, 20 Switch, and 30 RHers.
As stated by others, I'd use a 5/5 PC with a 5 pull setting. With the 5 pull setting, if your LHspecialist gives up a hit to his LH counterpart, he won't be left in to face the RH batter usually. If he does get his LH counterpart out, then he will probably be left in to face the next guy who is probably a righty, but you can at least take solace in that there isn't anyone on base in that situation.