If you had OE fogs then they already go through a relay. You can just splice into the wires behind the bumper.
Aftermarket fogs will likely use a 55W bulb just like the OE and you'll be fine.
I replaced the OE fogs on both my XJs (92 & 99) with Hellas and never had a problem with the stock wiring.
I suggest you get the waterproof crimp connectors for your splices. After you crimp them, you heat them and it melts inside making a waterproof connection.