About the metal, plastic boxes, I like to use the Slater Gray boxes with Quick Clik 6/32 screw threads. I like to use metal round boxes for all my ceiling fixtures and smoke detectors. The gem boxes, I like the 2 -3/4" deep, for most single ended romex runs, it gives you a little more room. It's allways good to go one size larger with the plastic nail on's. Instead of 18 cu. go with the 20 or 22cu. Not that much more in price. Have a great and safe weekend.
THOMPSON ELECTRIC
