1.
http://aquacomputer.de/downloads/aquaero5_aquabus.pdf see the connectors on right, the middle one is split and used with various MPS devices (including flow senor or the one integrated with pump)
2. I've got 1x DDC 1T and it produces so much flow that It hits limits of my MPS 200, yet I use lower speeds of the pump (~70%) and it stays around 160 liters/min. So you should probably go with MPS 400, but all depends on the restriction of the system.
Remeber to put the flow sensor in the 'end' of the setup, and it needs some straight hosing before and after it to work properly. Do not install it just after /before pumps. The best spot is the location before water getting into the reservoir.
6. My AquaComputer 5 Pro also had small heatsink, and it can handle voltage control od the DDC 1T via voltage (~1A), so it gets hot. I have noticed that even the slightest airflow really helps to dissipate the heat, I'm quite surprised that it cools so well with so low airflow, though.