I agree with this ,dogs that are regularly aggressive to others are an unmitigated nuisance,and aggression towards other dogs has no bearing on their working ability.
Two of the hardest most courageous dogs ,I have known ,[both from the Cumbria area ],owned by a very well know lurcherman], were completely chilled out and placid around other dogs,one of them , " Cougar", was a monster on Deer and Fox, had teeth like an alligator ,yet was a sweetheart in canine company,[unless attacked].
Its as though they know what they are capable of and dont need to keep posturing,and showing their teeth.On