I don't think I will paint this. The only really bad rust is on the tope tube and that will be mostly covered by the brake cable anyway.
That is a bit ambiguous! I can't work out whether it means you are happy to touch up the paint under there, or you are happy to leave the rust there? Just to clarify, I don't think anyone is suggesting that the rust should be left to itself, least of all where it is hidden by cables or components. It is localised to a few small areas of the frame. In those locations, I would clean it up, and, on the tubes, if it has
gone deep, give it a probe here and there with something sharp, just to make sure the rust hasn't gone right through the tube. (hopefully not). Then optionally some rust neutralising product as described by Elysarian, definitely primer and paint. You won't need much.. some wet-or-dry paper, a couple of tins of Humbrol, brush, thinners. It doesn't have to be pretty- you can always flatten it later with wet-or-dry and go over with another coat if you feel like it.
The Campag. stuff has come up nice. That's a Nuovo Gransport front mech... functionally identical to Record- visually about identical with the old Record cage without the holes, apart from the lip at the front of the outer cage-plate.