You certainly can. Just space them about 1/3 in from the edges. I so prefer the nail over the heating core.
I just made a 12x18 on Friday for the first time and I put a flower nail in the center of the pan and baked at 325 and the cake turned out perfect
in 15 yrs.. i have never used anything.. bake at 325 f.... alway done. always comes out of pan great..
Same here except I've been doing it for 30 years.
A commerical baking sheet is 18x24. Cut that in half and you have two 12x18 cakes (i.e. "half sheets"). Cut the 18x24 in quarters and you have four 9x12's (i.e. "quarter sheets"). That's where these terms come from.
I've noticed the grocery bakeries around here are putting out 11x15-ish cakes and calling it a half sheet. So when your client complains that the grocery half sheet is cheaper than yours, you can reply that it's because it's not a half sheet .... it's smaller.
But my closing advice is don't get trapped into using those terms. I refuse to use them. Most people have no idea what it is they are ordering. I had one guy who wanted a price for a full sheet. I asked how many people he needed to serve. Ten. He was serving Ten. I asked, "Then why are you wanting cake to feed 100?" 
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