
Border Terriers should be a working terrier, but their temperament is supposed to be much more social than most, not sharp, as they were supposed to run with hounds and so a quarrelsome dog would be unsuitable.
They are a great breed, and seem to fit in well with larger dogs, many people in my breed have owned both. They aren't overly active (would never think of them as hyper) and hard work in that way at all.
I know some good Border breeders and my friends Mum had a retired bitch from one of her friends, when the last Yorkie died (of the GSD Yorkie pair). She needed a robust small dog that was going to cope with visiting grandchildren/great grandchildren.
When her main carer (friends Dad) died she came to live with my friend for her last few years who died only recently, she got on well with my friends Doberman, and of course her children, not being as reactive as some breeds.