Can I use 18g cord for one light bulb?
18 gauge wire is commonly used in mass-produced lamps. I don't see a problem with using it for single runs of 60w max. It's rated for 7 amps at up to 50 feet. You'll draw about half an amp. 

However, the connections you make are perhaps more critical. Be sure to use sound methods when bringing the wires together at the source.