Hi Michael;
I'm into 3-1/2" gauge, but should be the same. Yes, bolt the frames together, to angles on buffers, and directly into tapped holes in frame stretchers. One can braze/weld the frames together, but you need to be sure of your bracing when welding, and I don't think it saves much time, if any.
On my latest 2 builds, I use metric cap screws onto the buffer "angles" rather than modellers bolts, as I think one can tighten them up more, and less likely to break. After painting, and tucked underneath running boards, behind steps etc, they are hard to see.
If you don't like tapping into blind holes for the stretchers, one can drill holes (say, 6mm) across the plate, then drill in to the sides meeting these holes. That way, you can tap easily, and can see the tap coming through the drilled hole. I've not bothered, but the DB23 (3-1/2" gauge 2-6-2) plans from the Netherlands show this.