Second dog as a compagnion for my CSW

[Cowboy Bebop]

Hi all,

I'm currently visiting breeders for buying a CSW (bitch) next year.
Because a CSW is not recommended to leave alone while working, we're looking for a second dog.

We currently already have two options: a fox terrier (male) and a hybrid Tibetan terrier x dwarf schnauzer (my wife's dog, also a bitch).

The problem is:
1. The fox terrier was adopted and did not receive proper upbringing or socialization because of his previous owner. His temperament is rather anxious and cowardly which causes him to bite strangers who approach too near or when they turn their back. He has food aggressiveness and also pees indoor.
So, we are afraid his unstable character might negatively influence our CSW pup.

2. The hybrid Tibetan terrier is our well loved dog. She's well socialized and stable. The problem is that we're afraid that a TSW might be too rough with her while playing and possible sex aggressiveness. She only weights 11 kilos / 24 pounds and is already 7 years old. So we would like to spare her.

Based on the characteristics of the CSW, what dog would be the best possible companion for a CSW during the day, regarding breed and temperament?

We currently have the option to adopt a 5 year old border collie. He's stable, socialized, a price winner in obedience competition. Is this a good match or are there any other dog breeds that are known to be a good match with a CSW?

Also, we would like to avoid any fighting dog breeds (pitbull, etc ...) since we have small children.

Kind regards and thanks in advance,
Bebop