I would not use Rhetta (except for, perhaps, a literary character or pet), personally, but I think that it is a set aside, overlooked, underestimated gem. Depending on how people choose to pronounce it (whether the "t"s are sharp and precise, or diluted to sound like "d"s - though you could still use it, either way), Red could be a fantastic nickname (I adore androgynous nicknames - especially ones that lean a little closer to the masculine side). The more I think about Rhetta, the more I like it. It is a bit like Rita (which I also like), and vastly better than Ruby. Hettie, Hattie, Rhea, or Ricky / Rikki could also be possible nicknames. (Along with Red, of course.)Rhett, male, is rather winsome and interesting, too.