Is it ever safe to sign with a non-SAG agency? And what exactly does it mean if they ARE a SAG agency?

It essentially means nothing nowadays. None of the real biggies are SAG franchised anymore. Sag has agreed that its members can be repped by non-sag franchised agents if the agency is a member of the ATA. If they are a sag agency it means they abide by sag regulations regarding talent, but most do anyway. Also, you can't list a a non-sag agent as your contact in your sag file. No big deal. Now, not being franchised is certainly not necessarily akin to being a scam agency. Do your homework with any agency, but only take into account sag status if there are other red flags.

SAG agencies can represent SAG and NON-SAG actors.

SAG agencies can only submit their actors for SAG commercials, TV and film jobs. SAG agencies can not submit their actors for NON-SAG jobs. They can submit NON-SAG actors for SAG jobs.