Most (if not all) fan splitters will run the fans in parallel rather than in series. If it were in series, they'd run slower the more fans you put on a channel as the voltage would be split - probably unevenly as you say.
I've not tried specific PWM fans on voltage control but certainly Noctua (fan manufacturer) say it is possible and they're not alone. I can see it being a problem if the fan has LEDs though as they may dim or not work at all. If you already have the fans and the Aquaero, you could always try it (unless specific fan manufacturer says not to) and see what happens.
Could you not run either 6 new fans on the cpu rad or 6 old and group them as one channel? Alternatively, are you aware of just how little gain you get from running push/pull? Have a read of some testing somewhere like
here as the added complexity, cost, noise and space may just not be worth it. Just a thought.
It is possible to add channels to an Aquaero. You can run a
PowerAdjust (standard version is fine - Aquaero does the control) to add a single voltage-control channel or you could add a
Quadro to add four PWM channels.