As long as the power draw is below 15 amps (or, say, 12amps if you want to play it safe and keep from blowing the fuse). The fog circuit has a fuse/label in the fuse box. As I was hooking up an LED light bar and didn't know what the capacity of the wires are, I kept it to 6 amps to that connector as the last thing I wanted to do was fry the wires.
If you want to have a big draw, then a relay is probably a good idea as you would be using wires that can handle the draw.
For arguments sake, I think you can do the following with the current set-up (using the fog light switch and wiring):
LED bars - One wire, 150 watt LED light bar at top (150 watts / 24 volts = 6.25amp); other wire, 126 watt LED light bar on bumper (126 watts / 24 volts = 5.25amp) - with this set-up, I can't imagine a stock alternator would be able to keep up; you'll be taking juice out of the battery! You could leave the headlights off, but gauge lights on, and still be able to enable the fog light switch and manage the power draw this way as the LED lights would easily overwhelm the halogen driving lamps.
Round lamps - 100w bulb, one on each side (100 watts / 24 volts = 4.2amp x 2)
If you just want to add fog lamps, you're safe. They all have fairly low draws.
Folks more knowledgeable on electronics can pipe in with more information (for example, determining how much current the wires can pull before frying
).
Jody
