Hi Ann
First, with the situation you have described, if you are looking for a livestock guardian dog (
LGD) I'd suggest one of the big white dogs from western Europe- Great Pyrenees, Maremma, or rarer Pyrenean Mastiff because they are more laid back, calmer, less reactive, and completely capable of dealing with the predators you describe. Follow the suggested guidelines for successfully raising a good guardian pup if you don't find a reliable adult. I'd also suggest a less intense herding dog. Border collies and similar breeds are really driven to do lots of serious work. More moderate herding breeds are better suited for helping out on the family or hobby farm. Personally I use a corgi but there are many other laid back herding breeds. There is no need to raise the two different dogs together; in fact it's inadvisable for a LGD to be raised with a non-LGD for various reasons. A large LGD can also hurt a smaller herding dog even in play.
Adult LGDs do usually accept pups really well. In any case, the herding dog and LGD need to become familiar with each other and the LGD will generally accept a familiar or family herding dog working with his animals as long as they are not aggressively chasing or biting them. Some LGDs do not like herding dogs actively working stock but most come to accept your dog and your regular routines with your stock. Some LGDs need to be restrained or contained when herding dogs are working stock.
You mentioned how we often see videos of LGDs and herding dogs working together with large flocks on open range or moving stock between pastures. Young herding dogs tend to be seen on a leash until they are responsible and trained. You can practice the same thing with your dogs until you are confident they are reliable together.
My corgis are familiar to my LGDs and they greet them warmly but I don't allow them to play or stay together. The moderate, laid-back herding dogs are also good family companions, and I choose to have them in the house. The LGDs are full-time guardians outside.