It seems to me that we need to look at this in order of your preferences.

1.  Protect from humans and small predators: The Sarpalninac or even your current Karst (younger) would to the job. You did mention something that I thought was very interesting. You would consider the Caucasian Ovcharka but was afraid that it was overkill. I do hear that kink of thing often but if raised properly it is not necessarily true. I have three COs on 10 acres and you have 7 acres - that is plenty of space for two of them to protect and enjoy the life with the animals. For the most part they will lay under trees and watch the animals and not bother anyone - until they have to. I may be slightly biases here but if you are considering the CO you should pursue it.  You are close enough to many great CO breeders so it should not be a problem finding a good working puppy. If you do decide to go with a Shar now and then add another dog later that is also a good move. The Shar is close to the Karst in behavior and size.  If you wanted to increase the protection you seek - then go with the CO.

2. Patrols the property: Most goog LGD will patrol once their boundaries are defined - your fenceline will have a worn path. My COs will patrol along the fenceline in the mornings and evenings and then find a high spot from which to watch everything. They alert easily and that enough may be sufficient to ward off any strangers.

3. Cold winters and "hot" summers: Any of the double coated LGD breeds are versatile enough to endure these climactic conditons.

4. Needs to accept an existing dog: Any of the LGDs if introduces as a puppy to your other dog will be fine until they grow up and establish dominance.

If you can, you should get a pair (male and female) of the same breed but of different bloodlines. You will have to bonded dogs to double your proctection.

Bottomline - I say get the Caucasian Ovcharka.