Maybe the reason why it's not working with more then 6 of your fans is, that the PWM signal from the aquaero has not enough power to feet more then six fans. If you connect one more fan, the signal level gets to low to trigger the internal pwm electronic of your fans. With some other fans the attenuation could be much more less and you could connect more fans then only six.
Pilo, you nailed it on the head.
It's been confirmed by Swiftech saying that they have seen it with Corsair fans that require a stronger PWM signal than most... so it was just that from the start and the mistery is solved.
Having just bought all the stuff, I'll not replace all my 16 fans with another brand, my budget won't allow me... So I'm monitoring only 12 of them with the aquaero, and 4 from the Mobo... not ideal, not pretty, but it works.
And so the PWM signal sent by the motherboard is also stronger that the one sent by the aquaero, and I don't know if there is a way for Aquacomputer to make it stronger maybe with a new firmware ?
At least now I know that making cable, crimping and such is not so difficult.
So for me the issue is "closed", thanks all for your inputs and insights.