This is going to depend entirely on the individual dog. I know dogs who would be fine and ignore the tortoise. I also know dogs who might try to eat it (not because they're mean, just because they're curious chewers, and well, dogs) Dogs can certainly be trained to leave other pets alone, but a lot of that will depend on the breed and temperment of the dog. Is the tortoise so large that it would be free ranging? Would it have a dedicated cage or room to call its own? If you could seperate them safely, its not much of an issue. If a service dog is a real possiblity, and you would get one already trained, I wouldn't be too worried. Those dogs are pretty highly trained to ignore most things, and the trainers would match your needs, like cats and a tortoise, with their choice of dog. A rescue dog, you could try a place that fosters their dogs out, so you have some idea of temperment, and then ask to give it a trial run to see how it goes. I wouldn't completely rule out a tortoise just because you want a dog.