Adult LGDs are extremely territorial, spayed or not spayed, THERE WILL BE FIGHTING.
The new dog needs AT LEAST two weeks of safe and confined acclimation time in order to understand that your property is her new home and for your established dog to know that she is not a threat. Either way, THERE WILL BE FIGHTING.
Does your established dog jump fences? Does the new dog jump fences?
Once you have kenneled the new dog for AT LEAST two weeks, it would greatly help to have her in a seperate pasture with her own flock of sheep, eventualy the two will get to know each other but there are so many issues at play,such as ... what type of temperament each dog has, is one obviously more reclusive and submissive, is one an obvious alpha bitch? Does one have a history of being a "team " player? etc. There are many "social dynamics" that are involved when introducing adult LGDs to each other which will help in determining whether or not they will be successful.
In example, if your new dog has a strong and confident personality and guarded solo at her previous home, there is a very good chance that she will beat the living tar out of the 7 year old if she does not respect the seniority status of the senior. I have three intact bitches, and four intact males. My senior bitch is 12 years old and I believe that she has lived this long because she never attempted to achieve alpha bitch status and always hangs in the background, she just does her own thing and she is an amazing guardian. However, my other two bitches exist well enough together on the same property as long as they have enough space to keep them occupied. I feed them seperate and I NEVER give them bones within eyesight of each other and I always try to have them with their own flock. When I do feed bones I make sure there are approx 3 or 4 bones per dog. Some "alpha' LGDs have a quiet confidence and are willing to mentor others as long as they arent overtly challenged, and some "alphas" can be bullies and will go cruising for trouble.
So in a nut shell, ( and to quote an old Forum sage).."it depends".