I have had 3 foxes from one big earth with 3 diffrent terriers. Tried a hard black bitch, dug to her. Tried another black bitch, dug to her and finally tried my black dog and he was dug to a dead fox in a really tight tight place. The fox had pushed it's self in to a tight block end with the noise of us digging the others out and had suffocated. All 3 dogs did what was expected of them, 3 foxes accounted for, happy farmer and happy terrier owners.
From my point of view the more time a terrier has to ground the better it will become through experiance. Obviously you don't want to put a pup in