TL;DRThe user should be able to willingly place pipes that connect different fluids (not acidentally, willingly).
What ?The user should be able to force the placement of a pipe that will connect different fluids. A ghost can be placed and will be revived and cause fluid mixing but before construction bots / without mods like Bluebuild the users can't cause this themself.
To avoid accidental fluid mixing I would suggest using ctrl-click to place a fluid mixing pipe or when reviving a ghost.
Why ?While generally fluid mixing is bad there are situations where fluid mixing is usefull. For example when fluid is used as fuel and one wants to switch fuels depening on availability.
Now that fluids can be easily be voided by clicking on a pipe system anywhere the damage of unintentionally mixed fluids is far less so I believe allowing for intentional mixing would be good and would enrich the game.