I don't think there is any real danger in connecting an "electrical" sub-d to a "fluid" sub-d. You'd just have a non-working connection from both sides.

It's similar to many devices having RJ-45 serial console ports, that look like network ports, and if you accidentally connect the wrong cable to it, nothing bad happens.

On the other hand, I could really see hybrid connectors as a solution, i.e. have 4 electrical pins and 5 fluid pins. That way you could have some signalling between the devices and not start pushing fluid through the tubes unless a "correct" device is attached.