Switches and fuses (or breakers) rated for AC only should not be used on DC. The problem is that DC is more difficult to interrupt due to the greater tendency to arc. Any wire will have some amount of inductance, and this will cause a voltage spike if a switch or fuse is opened. This voltage spike may be much higher than the nominal voltage and it may be enough to create an arc across the opened switch or fuse.
Lampholders probably will not have a problem on DC, as long as you don't try to remove the lamp with the power on. Incandescent lamps will work fine on DC.
I've never worked with high frequency AC, so I can't answer that part of your question.
