Yes, it takes awhile to be ready for that next loving heart to make it's place with you, but there's no disloyalty to Rex in anticipating that day with an open heart. It's good that you're exploring what type of dog you might want.
A Rottie that is raised with love and a strong willed owner is wonderful with children and other animals. Many of the working breeds tend to be more sociable. The Old English Mastiff is a calm and easily socialized dog. And I can't recommend a good German Shepherd highly enough. All of mine but my first (given to me by my grandmother, who rescued his mother) have been rescues, and Big and Bear were both adult dogs when we found each other. There are so many great breeds out there. One thing to remember, though, is that the large breeds tend to have shorter life spans.
Terriers of all types are wonderful, energetic, stubborn, willful, loving, intelligent and devoted guardians. There's no such thing as a small Terrier. They are just compact dogs. Inside each and every little body is a mind that thinks it's the biggest, baddest dog in the whole wide world. Did I mention they can be stubborn little beasts? Personally, I don't have a problem with Terriers and kids; if the kids don't respect the dog, well, they learn a quick lesson in the proper respect and handling of animals, and with the exception of the Airedale, Staffordshire and Pit Bull Terrier (which have very different - and vastly more tolerant - outlooks regarding the care and training of humans than their smaller cousins) the lesson, though graphic, rarely requires more than soap and water and "well, I told you not to torment the dog, didn't I?"
I got my first Terrier when I was almost four, and Mickey was a great companion. My Dad still laughs about watching his four year old daughter (me) teaching her dog circus tricks after a birthday trip to the circus. He says he knew right then that he and my Mom were in for trouble because there was no way a four year old should have been able to out-stubborn that little dog!
Since you love the Sheltie breed so, but are thinking of a larger dog, have you considered a Collie, Olde English Sheepdog or Bearded Collie?