TL;DRIt should be possible to connect pumps with different fluids on the input side to a common pipe.
What ?Per default pumps should behave like now. Whatever fluid type is on the input is also locked in at the output and vice versa.
But the pump GUI should have an option to disable that mechanism. The input and output sides of the pump should become independent. The pump would then only work if the output side has the same fluid type as the input side or no type.
I would suggest "[ ] Relax fluid restriction" as GUI option.