Is there any known issue of same-sex aggression in the breed?  As a breed trait.

No more so than in any other breed.

> No more so than in any other breed

but not all breeds are the same

Some breeds are known not to tolerate their own sex well, it's often recommended to only own a bitch and a dog in some breeds or to keep them separate when not supervised. I know of two instances where terriers have been killed by housemates!

Other breeds, like most sighthounds are known for being exceptionally good with their own breed regardless of sex. When I had lots of borzois I could put a male that had just mated a visiting bitch straight back in the kennel with 3 other adult males, 2 who were also stud dogs with no arguments whatsoever. In some breeds that would cause fights.

> In some breeds that would cause fights

When I was breeding I used to put all my stud dogs together to keep them away from in season bitches.  Never once had a fight.  Although they did sing in unison when they could smell that a bitch was ready.

When we had a bitch in season she was removed from the part of the house the others, male and female, were in.  We had to be leaping barriers but it was the only way we could carry on at such times and why we'd have our retired bitches spayed!!

I do know that in general (there will be exceptions), male terriers tend not to get along but I really don't know about Dallies.   I'd suggest it's up to the owner to do the introductions the right way and all should be well.     We started with two male Bassets and although they did have the odd spat, for the most part they got along just fine.  Not so much with a friend with 2, litter brothers and the only time they were ok together was when out on exercise!