How to Make "2d" Pieces of a Retopo Mesh?

[Mowen]

Hey all! Just jumped into this program today and curious how to achieve something I was hoping for. I am creating a model of a small fish that I sculpted in Zbrush and have moved into 3d Coat for retopology. 

I wanted to make the thin bits such as the fins and whiskers only one sided, to conserve polygons and to see how the effect turns out. So, in the end those pieces should be a flat plane rather than a 3d mesh, if you get what I'm saying! I've seen this done for strands of hair etc. This model will be tiny how I'm planning to use it so there's just no point in having them be full bodied.

I can't seem to finish the mesh how I would like to. For instance, since I can't hide the reference mesh (I've tried going into sculpt mode to hide bits of it, but it does not show up in that mode) I can't close the holes around the parts I decided I wanted to make 2d. Also, I'm not sure how to even handle the tail fin in a way to put the quads on and then either merge the 2 sides or somehow be able to create quads between the reference mesh?

I've also tried exporting the retopo mesh I have at the moment and loading it back in to continue making quads without the reference obj, but it was greyed out and wouldn't let me add any quads or do seemingly do anything.

First pick is with reference mesh, 2nd pick is me loading my exported retopo mesh back into 3d coat and trying to edit it.

Any ideas / information would be greatly appreciated!!!