Don't worry about it, use the steel brush on the steel pins. If you come across copper or brass pins, use the corresponding brush. After brushing, wipe off the pins with an alcohol swab.
polishing pins of NOS tubes
I have some NOS tubes with corroded pins. I read that you can use a short brass brush to polish them. You can even attach a brass brush attachment to a dremel and spin it, but that would be a bad idea for me as I'm very clumsy and would most likely bend the pins.
So I ordered some short brass brushes from Amazon and gave it a go. There was essentially no corrosion removed after brushing one pin for 10 minutes.
The set includes a steel brush. I brushed a couple pins with that for 5 minutes and they got shiny! However I'm wondering if this can scratch the pin and actually degrade performance of the tube.
Anyone have a recommendation?