A lil help, please, for a new member

[AB3]

Hello, all. I am new to this family and Rotts, period. I've been checkin' you all out for some time now. I have always wanted a Rott, but have not had the space (home) for one. Actually, I've always wanted two. But I now know for the wrong reason. They are beautiful animals which was one of the reasons I wanted them. The second reason was for "security, guarding, protection, etc." Again, I now know better. There are differences. Truth be told, I wanted a "mean" Rott. Wrong reason. Deadly situation. I also have found through reading these forums, 2 Rotts are not for the beginner, which of course, I am.
I will try to make this as short as possible as space is limited, but I have a great amount of questions. Many have been answered already. My first of many questions (I won't ask them all in this thread) is this. I am currently looking to purchase a home, if it isn't fenced, I plan to fence it before finding my pet. I want to purchase a 2-unit home, one to live in one to rent. I've come now to "overstand" this breed needs to be socialized, very socialized. What I want out of my dog is to "guard, protect" the home in our absence. I know the mere sight of a Rott would deter most would be intruders, but what about the others? My question, if my dog is socialized and only acts when threatened or commanded but is "friendly" to my tentants/other people how can I train him to know, "when you're with me these are nice people, so there is no threat, but when I'm gone, they are not to enter?" I want to ask more questions, but I don't want to use up all the space or take up a great deal of your time reading. Any insight would be greatly appreciated. As I have seen there are experienced members here and have been very helpful to others. I have taken a great deal of time (many years) to research and study this breed and believe, no scratch that, KNOW I'm up for the challenge they bring. This is a pet first, but "Do not enter my domain unless asked to" dog second. Oh, I'm sure you want to know, my family consists of myself, fiance, 3 children, 8, 6, 3 yrs of age. I've read they are very good with children and I know they MUST BE AT ALL TIMES SUPERVISED. Can I get one more question in? Is it too much to expect the dog to respect the children (8,6 yr old) commands? I know I must be Alpha and the children are mine and he should respect them, but should he/she take orders from them?