I prefer it as a nickname.
As for spelling, I'd stick with Ro. Roe makes me think of Roe v.
Wade. Also, Ro looks a tad more feminine since
Joe is usually used for males and
Jo for females - not that it matters tremendously or anything, though.
Other full names:
Rowena or Rowenna
RowanRosemaryRosemarieRosalieRosamundRoseRosaRosabelRoselleRosarioRosariaRosannaRosangelaRohesiaRominaRomanaRomaRochelleRoisinRoxanne
And just about any other
Rose name.