You can net foxes with a terrier that bays (barks) at the quarry, i've had some big terriers but they did all right to ground. Stick with your pup mate till it's older as at 42 weeks old it is still too young for quarry that will bite back. As has been said they fold there legs back so it's chest size that counts. If it's chest size is 9 inch then it might get stuck but that is a slight risk. The pup will only be chasing the other terrier as that's what pups do. Good luck with them mate