if you get a well bred, working stock patt. but most importantly, make sure it's kept busy working, you should have no problems, a good, working terrier usually won't fight with other dogs, terriers that want to fight other dogs, especially when on a lead are usually cowards, just my oppinion no offence meant.
How do you work that out?
do u mean if they attacked whilst of their leads they would still be a coward?