I am not going to be terribly objective here, especially as one of the beloved family pets in my history was a rescued (from clueless owners) Great Dane mix named Gracie! Black, not merle, but sweet as can be. I would probably not have any chance as resisting her, myself.

That said, are you sure you should add another dog right now? Perhaps you should wait until all four dogs are living together, in the same house - that may be a big adjustment in itself. Moving halfway across the country will be another adjustment, and both could be pretty stressful on your current dogs. I know, once you're in New Hampshire Gracie will be too far away ...

I don't think that any particular number of dogs is "too many" but that each canine/human family has its own dynamic to work out, and yours will be no exception!

Don't let Gracie's size be any determining factor - both rescue Danes I have known have been very sweet, not at all territorial, just happy to be loved!