Hrrm, that is gonna be complicated for In-World Mixing for sure, especially with Rivers. This is so definitely gonna be a thing I need to take care of later, lol. It is definitely gonna be possible to mix Fluids since there is no concept of registration for Blocks, so those can be created on the fly with whichever Stats are needed, such as 50% Water 50% Sewage, and Sewage and Water could mix further down the River so that the entire thing gets polluted evenly with like 99% Water 1% Sewage spread over all River Blocks.
Depending on how Fluid Flow Speed is implemented it could be an issue if Sewage is slower than Water, since that would mess up the overall Speed.
Yeah this is gonna be a huge Headache for Future Greg to figure out. XD