Mixing breed or keeping two different breeds generally does not make any difference what you should keep in mind that do not get the same sex dogs as fights etc can happen later on.
I would advise you to first get one dog and wait for atleast 2 years plus before adding another. Guard dog breeds are very demanding dogs and it is best to get one and train it properly etc. Why I say 2 yrs plus as the behaviour will keep changing in the sense it will keep testing you out and will require constant training and reinforcment of what it learns. Females are generally easier to keep and train apart from their heat cycles. Both males and females can guard just as well.
You must also understand your need guard dog or watch dog. They are two different things that people confuse to me one.
I would suggest go for a Great Dane or so who's appearance in itself will scare most people, and will be less demanding as compared to the breeds mentioned by you, but training is a must for all dogs. Guard dogs like the ones mentioned by you are not dogs meant for everyone or anyone for various reasons. This I am saying who's first love is Rottweilers and I have been keeping and occasionally showing them for over 20 yrs, time permitting.