I personally like
Ben as a nn, but think it's a little short by itself as a name. However, there are mothers who disagree with me & have named their sons just
Ben. Imagine! :-) I would use
Ben as a nn for
Benjamin,
Benaiah,
Ebenezer,
Bennett, but not for Rueben. I like that name enough to let it stand by itself; however, it's entirely plausible to use
Ben as a nn for Rueben.
As for
Isadora, I much, MUCH prefer it to
Isabella, and don't think it sounds frilly at all. I agree that it sounds like a strong name, but is more pleasant sounding to my ears than some other so-called strong feminine names.
Isadore on a boy is ok, too, but I can't think of any nn for him other than
Izzy or Dore. If you want a cool nn, you'll have to be creative!