OK so here goes... the pallet jacks you rent are usually the wide ones, but are quite inexpensive if bought used, and you wait for a deal. the really narrow ones (Jimbo has one, are rarer, much more expensive, and not usually rentable). However for moving a mill around a concrete floor, I much prefer to do it with 1" pipes (or 1.25" solid rounds), because then I am using a pry bar, and I get there nice and slowly with no drama. (Remember Janger's post above?) - on downhill slopes i use a pry bar and slide it down steel rails, I mostly use 1" angle iron, corner up.
My garage has 8' headroom (Or is it 8'2"?) and I have plenty of room for a full sized 9X49 BP clone. In fact, mine was on two pallets (making it 9" higher, and I had to lift it UP to get the pallets out...) I think for any mini-mill or even full sized mill, you can get it under the 7'8" of a standard stud height wall with single and double sill plates.
It is important to have the mill firmly touching the floor, and the levelling feet help to remove vibration and make even contact with the concrete.
Some guys put their mill on wood structures, but I've never seen a professional shop do this.