Hard clay huh?
Sounds like grading will be a priority. Which is probably best done with a box blade or rear blade on your tractor. I can fine grade much better with the reaer blade on my tractor than I can with any loader, and the old skidsteer that we used to have.
It sounds as if you should put a loader on your tractor for moving light, loose material and a box blade to grade and loosen up the hard clay. The skidsteer will dig in clay, but not easily. And not without teath on the bucket.
The mini excavator will dig on clay just fine. One note, If you unload your pick truck with a mini excavator, put some sheets of ply wood down prior to loading the truck. The bucket teeth do nasty things to a pickup bed.