Yes, normally a M6x1mm thread machine screw with pan head, with a screw length of typically 20mm, covers pretty much all rack mounting equipment. and will fit the captive nuts perfectly.
Of course with old racks made up from reused parts from other racks you will find the odd cage nut that is a US size, and I normally fix those by replacing the nut with a metric one, or if I have none left run a M6 tap through the nut to make a somewhat skanky M6 thread for the screw. I do have a small assortment of imperial taps and dies, but those are generally not used much, unless I am making a bespoke hole for a thread I will also make, and the size is right for them, or for some US made equipment that uses them, though there I also have a few recoil sets ( bought cheap as well, as 30 year old stock off the dusty shelf) to handle the common sizes.