Short answer is yes...that 12v outlet that you linked to comes with 18 gauge wire, really only enough for cell phone charging and other small current draws. Wiring it into the light fixture is fine for that usage.
Basic electricity: volts x amps = watts. So that 100w inverter that will provide enough power for a flat screen TV and DVD player, or a laptop charger, is delivering 0.83 amps at 120v, but on the other side it is consuming 8.3 amps at 12v.
What you did is fine for what it is, but it's not correct for any kind of inverter, even a small one. The best way to work this is backwards....start with what you want to power with an inverter. Look at the labels on the equipment and add up the total watts, this sill tell you how big of an inverter you need. Use the formula above to figure out how much 12v power that size inverter will draw, that will tell you what wire size and fuse you'll need.
