I'm working on a Y6 and I'm having some wicked yaw problems do to some slightly different front motors. (Red is top, black is bottom)
They are all 530Kv motors, but the as you can see from the drawing the top is red, and only the top front 2 are 250w 5014 and the other 200w motors are 5010.
I'm pretty sure I can cancel out the torque from the more powerful motors if I could reconfigure the motor inputs and I saw the motor mixing page https://code.google.com/p/ardupirates/wiki/motormixing
I also know that I could just move both larger motors to rear coaxial boom, but I have it set up where it the center of gravity is just under modified center of lift due to a forward mounted brushless gimbal, it's more a T6 than a Y6. So losing that additional lifting power at the front, I will have to shift the cg back further, but might only be done by extending the rear boom and shifting the 525 g battery back, but it will spread out my weight distribution and increase the inertial momentum.