Search TT and web for JayCee stringing method. He strung for some pros and supports string X higher than mains. Theory is the X have to be tighter to account for friction and increase M tension due as X are pulled. If M is strung at 55 lbs, and you pull an X thru it, it actually increases the M tension according to JC. JC strung for Bagdatis among others.
Other say when hybrid with poly main say at 50 lbs, a soft string cross should be a little higher to account for difference in stiffness. There was an old site where luxilon stringers answered questions, and I think they worked for lux. They recommended stringing mult/nylon/gut crosses a few lbs higher than poly mains. I think the recommendation was in the range of 2-3 lbs tighter for the soft cross.
But, some pros with gut crosses and poly mains actually drop the cross 2-3 lbs to allow for sliding.
So, there is a theory or a pro out there to support about anything you want to do.
for me:
1. same string in M and X. I recommend the same tension or drop the X 2-3 lbs below the M.
2. poly m with soft X: soft X 2 lbs higher than M.
3. soft M with poly x: poly X 3-4 lbs losser than M.
But, I am going to experiment with poly M and soft X with soft X 2 lbs below M just for fun. I don't think I have tried this yet.