Cog-to-cog interference is possible if the spacing is too narrow, but so far doesn't seem to be a critical issue. 5-speed pin width is 7.2mm, so theoretically could function with >3.6mm spacing. Closest current spacing is 3.85mm.
Except that the spacing refers to center to center and does not account for the cog width of ~1.9mm
Trying to run a 5 speed chain on a 9 speed won't work because there is only 7.2mm of space for the 7.2mm chain. When a chain rubs on the next larger cog, the ramp causes it to shift (downshift) for part of a revolution until the rd forces it back onto the higher gear. This happens with each revolution.