To fly right seat under 135 you still need 135 mins, you just don't need 135 PIC mins. If you have a commercial multi with instrument rating you're good to go for a 135 company (that flies multi engine airplanes). The trick is finding one that has a SIC program. Where I work, we don't have any SIC program for our King Airs or our 310s. Many operators fly these types of airplanes with only one pilot and an autopilot. Good luck.