If you can wait until the end of October, when the Aquaero 6 Pro and XT will be available, you probably don't need any other equipment like the Poweradjust anymore. Just the Aquaero 6 will be enough to control all your fans and pumps.

New: aquaero 6
http://shop.aquacomputer.de/product_info…roducts_id=3092
http://shop.aquacomputer.de/product_info…roducts_id=3093

It depends on what pumps you are talking about. 2 Laing D5's can draw up to 4 amps and would be way too much for a single channel, but 2 Laing DDC-1T's would draw less than 1 5A and could easily be used on one channel.

Pump speeds can be controlled by temps. The poweradjusts show up as just extra fan channels so anything you can do with fans you could do with pumps on PA's. The USB pumps can be a bit different, the D5 usb can def be controlled via temp based curves too though.
Pumps in series (Laing pumps) are easy. The pumps can be run at the same speed or completely different speeds, it makes little difference. The flow rate produced in the water loop is determined by the combined pressure of both pumps vs the combined flow resistance of all components.
All the pumps do is spin the water inside them very fast. That centripital force makes the water want to flow out the outlet. Because even at minimum speed the pumps still spin much faster than the water coming can travel in any loop, its totally possible to have pump 2 at full speed and pump 1 at minimum speed. The flow rate is nearly exactly the same as if pump 1 is at max and pump 2 is at minimum.

Most of USB devices can be run with only the 4 pin aquabus connected after setup with USB. Although some functions will be limited that way and settings can't be changed. It depends on the particular device.