You will enjoy that combination.
I agree with the 'not backing up' when there is brush bent forward under your tractor. The belly is exposed a bit, and even some wires and hoses to be considerate of while bush hogging.
But you can bush hog into areas of brush by backing into them with the mx6.
Break in shouldn't be anything special, other than the manual pointers. I wouldn't run full rpm (for the Mx6) until 10 hours or so. But I usually don't hog brush at full rpm anyway. Probably around half full. If I am cutting tall grass, then I crank it to the rec. PTO speed, as it does a better job of getting the grass cut. But lower rpm on brush seems to do sufficient, and I don't think has the potential to throw stubs around so much.
Setting the height of the mx6 may be the first challenge. Let it ride on the rear wheel with some slack to move up and down, independent of the dips and doodles the tractor does. Adjust the height of the front of the deck with the 3pt arms. I have my top link adjusted so at full height of the 3pt arms, the rear wheel is about 6" off the ground for 'travel' mode.
Lots of help here, so holler if needed. Also, a search of rotary cutters, bush hogs, etc. will give you several threads with good information and even good questions others have asked.