Awesome, I always wanted to make a tank. When I made my fairing I used the blue foam from Lowes. Before you go and apply your resin and plies, test the resin on the styrofoam, I have a feeling that it will eat it as well. Since you are making a male mold and laying up plies around it, you will get a rough surface on the outside since it isn't tooled. Are you planning on vacuum bagging the part? Might help with compaction of the laminate and possibly the outer surface finish. Also, is the clear coat a UV protectant? If so, what are you using, I plan on heading to the boat place and see if they have something. I just made a dashboard for mine out of CF. Trimmed and drilled it today on the mill. Needing to prep and clear it sometime.
As far as laying the fiber down, if it is a plain weave it might make it without darting. Twill would lay down nicely as well. In my opinion, I don't see why fiberglass or carbon fiber should be different when laying down. The thing that really dictates how the fiber will lay is the weave of it.
Sorry, just thought of one more thing, you might have to use 2 plies of CF on the outside. Reason is that you might be able to see the fiberglass show through the edges of the weave.
I'll be following, so please update progress when you can. Good luck.