We frequently used those RD240s with a ton of different RAID cards (including SAS3 models) without issue. I believe I even ran some drives at 12Gb/s speeds, but it was more for the hell of it, not actual testing.
On the PSU connector, that was one area we never went into on these systems. For powering certain PCIe cards, what my solution did was use a male SATA power adapter to molex, chop some of the pin shroud off on the adapter and insert it into the drive backplane. That was the easiest way that was also removable if needed.
Something like those, with the side pieces chopped off to allow it to slide into the backplane. This has to be carefully inserted with the utmost patience since you aren't guided into position with the DATA side. You need to make sure you are sliding it in fully seated to the power side, and not bridging any pins. Have it misplaced a mm or two and you fry the backplane.