I don't know what setting the number of shells to 0 does in Replicator-G. My guess is that tells Replicator-G to do what ever it thinks makes sense. In Slic3r (if I understand what a 'shell' is in Replicator-G) the equivalent term would be perimeter. You can set how many layers the perimeter has.

For people that use OpenScad, Slic3r is the usual slicer. But it depends a lot on where your head is at. I don't use it, but the people I interact with feel Replicator-G has fallen behind. The last release was over 2 years ago. If you are willing to switch to Slic3r and PronterFace, I think you will find more people to help you in the forums and probably, you won't need much help because both of those tools are pretty good.