Dog compatibility and behaviour

Question
We have a mixed breed border collie, staffy and probably a few other breeds too. She is about 2.5 years old and we have had her since she was about a month old. She has a beautiful nature, is playful and wouldn't hurt a fly. She doesnt not really socialise with other dogs but on occassion other dogs squeeze through our gate and Jeisa just loves to run and play with them. We would like to get her a friend soon and my partner would prefer to get her a friend that could act as a type of guard dog. At the moment Jeisa will bark if she hears a noise, but if a stranger came up to her, she puts her ears down, wags her little tail and is quite happy to meet them. My partner would prefer a dog that will not bite but will at least growl if a stranger approaches. I do not want a dog that will bite or be savage in any way. And also I do not want a dog that will encourage Jeisa to become aggressive, as her nature is purely sweet. We have discussed the option of getting a Rottwieler but I dont know if this breed would be appropriate. I have learned that this breed is about double the weight of Jeisa and I dont want her to be dominated over by the other dog. Any advice on the matter would be appreciated. Also Jeisa gets quite upset during thunderstorms. It takes a lot of work to calm her down. She doesnt like loud noises but thunderstorms tend to give her massive anxiety. Is this behaviour that can be changed? If we get her a friend, I would not like her to pass on this behaviour. Thank you.

Answer
I have a sweet border collie and she hates loud sounds as well. She had never gotten a friend so we took her to the dog park a few times and she would run around and bark with them but never had anyone to be with at home. When hurricane Katrina came we wanted to adopt a dog that was left alone and we were driving back from a meeting when I saw a massive white dog tied along the road. We pulled over and it was a sweet brown and white American Bull Dog who was what everyone called as a bloody skeleton. We took her home and now the two are the best of friends. They have their issues of who is top dog but my Maddy (the B.C.) always has kept that title. Maddy loves people and Neeta (short for Annuneeta the A.B.D.) is the protector of the neighborhood. The thunder issue always seems to get worse with age. Theres now way to fix it as far as I know. Hope this helps and feel free to follow up.