so next week i'm making a 10" cake with a mermaid tail sticking out the cake, so i was thinking of making the tail our of styrofoam but not so sure about the fin. I'm afraid it will be to top heavy...any suggestions. 

If the tail will be styrofoam, you could make the fin out of foam craftboard that you get in the posterboard section of the craft store. You could build the whole thing in advance and stick s few sharpened dowels like spikes into it so that you can plop it on top of the cake and spike it in there. If you have three spikes that should attach it pretty well, especially if there are little splashes of fondant coming from the bottom and attaching to the cake and the tail like i the photo. Those will glue them together pretty well.

I would actually use RKT.  I like everything to be edible what goes on my cakes. Cover it in fondant mixed with cmc to stiffen it. 

thanks i'll try that.