I would NOT recommend you hook anything to the light's ballast. The ballast was designed to run the lights, not extra equipment. Think about it... would they spend extra money to put a ballast on there that would overpower the lights enough with power to spare for a rainy day? Nah. Even though these 80mm fans are small, they still take a lot of power to run them... especially if you get 2, 3, 4, or more.
Plus, if a fan blows out, you don't want to risk short circuiting your lights. I had a PC
fan hooked up to the same outlet as one of my 6 hard drives, when the fan blew, so did the hard drive. Same concept. The smart way is to get an AC
adaptor 12V DC
output and wire it separately. Less chance of something going wrong.
Secondly, that fan will only push 24 CFM which IMO
is next to nothing. Go with at least 30+. Plus... these things are $18.95 each? My total cost from CompUsa was $12.60 for 3 fans, and shipping. 8)