1.) The aquaero is programmed as an HI-Device and acts like a keyboard or mouse. So you won't need any drivers but of course you will need software to take control. We do not provide any software for Linux but one of our customers has programmed a tool:
http://breakbe.at/development/aquaero/
2.) Three of these fans on one channel will not work with a regular aquaero. The maximum load for each channel is 10W while the overall load of all channels combined should not exceed 30W. If you take the variant with powebooster each channel can do 15W (= the load of the mentioned fans) and the overall load is 45W.
3.) None

Since it is a stand-alone device with its own CPU and operating system you would not need a PC. Of course if you want to use the software every PC with USB and Windows + .NET Framework will be enough.
4.) You can download the SDK + documentation
here.