Is there a consensus on mixing fans in a case?

RAM used is the new Crucial Sport VLP series, which will fit under the front fan.

There is nothing to stop you but if you are planning on connecting them to a single connector make sure they don't go over the rated current for that connection
In my opinion which I'm aware you didn't ask for lol I don't think adding another fan is going to improve the temperature significantly maybe 1-2 degrees if your lucky but hey you should test temps without the extra fan and then with the fan and let us know just out of curiosity

My motherboard has a 4 Pin CPU fan slot which I'm not using and a 4 pin chassis slot above the heatsink. One of which I'll use for the second fan XD.

I'm pretty sure the cpu fan connection will take both fans on the cooler but I'm not sure would it take a third

I'm not using either, so...