Mixing Size is not the worry, Mixing ANYTHING ELSE is the problem. If both SETS have identical CAS Timings e.g. 9-9-9-24-2T then you'll in theory be okay; edit yeah duh too the Frequency, CAS, voltage, brand, IC's...
The P67 seems to be REALLY PICKY when mixing even the SAME sets which is why I linked what I did above, I know they work