Google can't count. The numbers Google gives you can't be trusted at all, and it can be proved easily. I suggest you never make any decisions based on the estimated number of hits Google gives.
As MisterMicawber said, only "They are fools" is correct out of context. If you google for "They are a fool" you'll get several results as well, but they are only acceptable in ce
"They" is plural, meaning more than one. Therefore the "s" on the end is appropriate. The "a" needs to be removed since it is used for singular meaning just one.