I have mixed fans without a problem. It is really no different than have unmatched intake vs exhaust case fans. Air compresses and sure there is some minor speed up in push pull, but i have yet to see where mixing fans is harmful. I suppose if you put a 5000rpm delta behind a 500rpm fan you might see something entertaining, but not a few hundred RPM. I think I tried 3000rpm UK3s with yate loons one time and still didn't see any problems. When we are fighting against resistance the blade is slipping if you want to think of it that way. We usually see less than half of max air flow. Two fans in push pull can increase that by 20-30%, but it is still not the maximum flow of one fan in open air.
Anyhow, I haven't been able to find any historical example of where mixing fans is bad if you keep them roughly similar and I have tinkered with mixing some fairly extreme different fans and still got the same good result.
Now the kit is set up (finally went for XSPC EX360 with push-pull, although only has the push for now, the pull fans are still sitting in its box).
I noticed the water in my clear tubing is getting a little cloudy. I've put in a silver coil and some additives for the water, and washed each component before I put it together. Would this cloudy problem be normal? Or did I mess something up?
Just saw this thread. I am no expert, but there are long threads here about clouding and "plasticizer", which, as far as I can determine, is an element in some tubing getting leached out due to an unknown combination of chemicals, pH, and gremlins. I'd suggest searching the forums with that term.