Hi Doogz,
I confirm that the QL fans are compatible with the Core. I have done exactly the same thing. I took out the ML that came with and bought 6 QL (4 140s and 2 120s). The only thing that you must be aware of is that all fans on the same channel must be of the same type (QL, ML, LL etc).
Let me know if you need anything else.
Best,