How to use thinner insulation than the steel studs?
If paper-faced insulation is available, I'd consider using an adhesive caulk to hold it to the studs. The product we use for polyethylene vapor barriers here is called "acoustical sealant". It remains very flexible, so it's less likely to cause bulges in your drywall. 

Alternatively, use a spray adhesive on the back side of the drywall that's already installed. Press the fiberglass onto it while it's tacky, then install the opposite drywall face.