Look, here's the deal:
If you get an interview for a large 135 or 121 op and you have SIC logged in an airplane that is SP approved by the FARS, you're in some hot water already. Maybe you can pull out your Ops Specs and say that it was approved by the FAA and they'll let it slip, but probably not. Best case scenario, they'll go through the rest of your logbook with a fine toothed comb and find something else you screwed up and nail you there instead. If you somehow logged everything legally, you'll get a very difficult interview.
Why are you logging this SIC time? To impress your buddies in the hanger, or as part of an effort to move on to 121 or large 135 ops? If you're only trying to impress your buddies, log all the SIC you want. If you're trying to move on, then don't log SIC unless it's required by the FARs. Basicly, if you're flying anything smaller than a 1900 (in passenger ops), then you shouldn't have any SIC in your book.
Incidentally, I have never seen an application that has a spot for piston or SE turbine SIC (other than Military fighter SIC, if there is such a thing as a 2 pilot SE military jet), so why are you logging it? There nowhere to put it on the app!
All the good experience you get flying right seat in a Caravan or light twin, yeah, it is good experience. Talk it up at the interview, let it help you in the CRM scenario questions, but don't log it as anything more than TT or MEL or actual instrument.