I have same setup, but I have three of the rechargeable square dehumidifiers in addition to the GoldenRod. The rechargables have a blue indicator, and when it turns pink, you plug it in until it turns blue (mine takes about 6 hours), then put it back in the safe. Might be a little overkill, but here in South Louisiana, there is plenty of humidity.
20, get them in a safe asap. The "cabinets" are easily broken into and your firearms are gone. Go with a fireproof safe, it's well worth the cost. As for rust, I'm in Az so we don't know what humidity is.