Can I log SIC under 135 IFR?

Yeah, like Dan said, either one of the two of you in the cockpit is able to log SIC/PIC time (IAW), but you need to be checked out under 135 regs. That said, since the airlines you're applying to (assuming regionals like Mesa or Skywest) will probably want to see just PIC time, since you have to get that time to land a job with them. Assuming you want to go the airline route, it might be best to just wait and log the PIC time, because the SIC time you log under 135 will count as PIC, but you technically have to log it as SIC time. So, it would probably be better to wait and log PIC time since that will be the best thing for them to see.