Well firstly I think Harriet, Agatha, or Ramona go the best with your Eloise and Beatrix. I think all three of those options sound pretty good with Sutton. Harriet Sutton, Agatha Sutton, Ramona Sutton; pretty good actually.
Other gender neutral/surname names that aren't too common:
... And really there are many more.
Do you have a maiden name you wish to honor? Someone in the family? I saw a writer who's first name was Olsen, and that made me think that maybe when I get married someday I name my son after my maiden name, with the nickname of Olly. It was just a thought though, but people do it!
oooh, I ADORE the mn Grey, especially spelled with an e rather than an a....if it's a family name you should totally use it! (Makes me think fo Agnes Grey, which I am currently reading... :)
Ramona Grey sounds lovely with your existing kids. That or Harriet seems to fit the best, era/style-wise.
I also like Theodora Grey, Agatha Grey, Rosamund Grey...everything except Guinevere Grey. But Guinevere Murray is fun.