I agree that a B-B would be best.
As for guard rails, the ones that are in place are there for the purpose of keeping derailed equipment from wondering too far from the track and going over the side of the roadbed.
The type of guardrails that are mentioned to help guide the #21 from coming off would need to be spaced more like the guardrails in turnouts - real close to allow it to guide the back of the flange around the curve.
I had to use this type of guardrail in a 13' radius curve for my 4.75" Sumpter Valley 101 model to keep from derailing. It is a pain to install but it does work. My 2-6-2 steamer did not derail on the same curve.
FWIW.
Doug vV