Not to give you any more options BUT ( ;)) Im a big fan of MOSFETs personally. They are fast enough that if you were to choose to do some PWM to slow the fans down (to make them quieter) at a later point you could. Here's a schematic that would work for you:
http://www.csulb.edu/~bevans2/schematics/mosfet.jpg
You would want diodes as shown for back-emf protection, the fans are 'noisey' like that. But with its higher current you could easily run all of the fans off a single MOSFET. The fans would be wired in parallel, or all the reds connect together and all the blacks connect together, with the reds going to the +9v on the arduino and the blacks going to the 'drain' pin of the 'FET. (Keeping in mind the diode has to go across the + & - sides of the fan.) So yes, you could use the 9v pin on the arduino, since it is really just a pass through, whatever goes in to the plug is what comes out there. (ie If you were to use a 12v supply it would be 12v at that pin instead of 9.) Otherwise regarding safety, use a good regulated switching power supply (my preference) and nothing should go wrong.
BW