In my book, it's OK to mix the grease ... but ... I'd clean the old out of both CVs just in case there is any contaminates (water or abrasives). If there's any grease left on the new components, I wouldn't worry about it.

I didn't even use the grease that came with the MechaTech's. I just got a tub of synthetic axle grease from the parts store & used that:

a. I didn't know what kind of grease came with the boots & I wanted synthetic.

b. the grease that came with the boots didn't look like nearly enough so I was going to have to add to it anyway.