Powering arduino and 12V Fan from same supply
The board will take 12V, but the regulator will have to dissipate (as in heat) the excess power.

However, how much do you trust your power supply to not exceed the 12V?

The same applies to the other loads.

About max current, you should do the math for both typical case and peak case. Ex: the fan will draw much more current if for any reason it is stopped.

If you use only 1 transistor, you will get both fans to behave in the same way, because you will be using only 1 control line. Is this what you want?