At 6-3 190, it is not that the 25 year old is too small to be a starter, but it is hard to imagine that after being in the bullpen since being drafted (one start in 2010) that he will become a starter.
Since he pitched in the Arizona Fall League, we have some Pitch F/X data from one of his outings thanks to GameDay (Brooks Baseball didn't have a profile or data on him from what I could see):
His fastball bottoms out at 87 MPH (once in 2.1 innings of work), and he sits (or more accurately, sat) 88 MPH-89 MPH. He also hit 90 MPH twice.
The changeup he throws doesn't provide much speed differential at 87 MPH, and I believe he only threw it once (the movement was different enough that I don't think it was the fastball). That most compared to Colby Shreve's change in Arizona Fall League Pitch F/X data. Shreve, a prospect with the Phillies, is considered to have a good change according to reports.
The main breaking pitch he used was a 80-82 MPH slider. It is close to Phil Hughes and Justin Marks in movement. Marks' slider has been described as a "wipeout slider" and as a "good pitch". Phil Hughes' doesn't throw his slider much at all (2.3% for his career, didn't throw it at all from 2009-2011 according to FanGraphs), but it does have a positive run value and opponents have just a 59 wRC + on it (but classification problems abound, especially with the velocity difference). According to GameDay, he threw one 80 MPH cutter with similar break to his slider. I'm guessing it is his slider.
In 2012, Rearick split the season between A + (45.1 innings) and AA (24.2 innings). He was about a year and a half older than league average, but at least he was dominant (1.46 FIP, 2.48 SIERA). He found AA less friendly, especially when it came to home runs (3.22 SIERA but 4.29 FIP). Obviously everything is small sample size, but the Rays' AA home park did play hitter friendly (but he also gave up more homers on the road). I wouldn't put much stock in those numbers, though I find his lack of large platoon splits interesting. His strikeout and walk rates are still solid against righties (and are really excellent against lefties), the big difference lies in his ground-ball rate. This does lead me to believe he would have bigger splits in the upper minors/majors (if he ever makes it there).
Since there aren't any videos of Rearick (that I could find) readily avaliable, I went back and took some video off of MiLB.TV (on the Pensacola Blue Wahoos broadcast. Billy Hamilton is on first when the video starts):
He has also been known to throw a 70 MPH curveball (though that may be outdated). As the prominent Rays blog DRaysBay notes "Scouting information for Rearick is difficult if not impossible to come by". He is long and lanky, and delivers the ball from a prominent 3/4ths arm angle.
The delivery doesn't look like one of a LOOGY or left-handed specialist, but instead of one like a starter. I don't think he is limited to just being a left-handed specialist because of the arm angle, though you would like for him to face more lefties than righties because of the ground-ball trends we noted above. The simple delivery could allow him to eat up innings in more of a low leverage swing man role, and the good command (at least from the numbers available) would help in this. If I were the Padres, I would be aggressive in assignment (of course a lot will depend on how spring training shakes out) and assign him to AAA Tucson to compete against players that are closer to his age. In AA, he will be a little old, and with the pitcher friendly San Antonio park, the numbers won't be trustworthy. Whether he is able to keep the ball in the park against more advanced veteran hitters in the PCL's hitter friendly parks will determine to me whether or not this pitchability lefty has any future value in a MLB bullpen.