Why not? There have been many pitchers through the decades who have done very well with a sinker-slider combo, and a lot of them were and are sidearmers. For example, the Yankees' Spud Chandler in the late 30s up through 1947. And he was a starter to boot. And as for relievers, there was Jim Brosnan who did very well with this combo when he pitched for Cincinnati in the 60s. So if you've been thinking about adding a slider to team up with that sinker, go ahead, and being a sidearmer you would very well to pick up the crossfire---I can tell you, a crossfire slider is one of the most unhittable pitches there is. Go to it!