I've done a lot of research on this and there are a lot of people making this type of cake but I still wanted to ask a few more questions. I've read you can put royal icing, buttercream and molding choc. on RCT before fondant but I'm more likely to use one of the first 2.

Which would be better?

If I used Royal would it adhere to the fondant?

If I used BC would it make the RCT soft and cause it to sag? 

It's a grooms cake for about 100-150 attending.

About how big should I make this cake? It will be a round 2-tier cake. This is what I'm going for.

<a href='http://blog.pinkcakebox.com/bass-fish-rehearsal-cake-2008-04-26.htm'><img src='http://images.pinkcakebox.com/cake630.jpg' alt='Bass Green Fish Cake with Fisherman Hook'></a>

Ive made a few bass head cakes... I usually use white chocolate ganache, because I have it all the time, and it sets up quick in the fridge. After chilling, I rub it down to smooth it, add some here and there, and re-chill. Sometimes ill use modeling chocolate, then, to add details under the skin, like gills, lips, etc., then wrap it all in fondant, emboss the necessary texture with my dresden tool, then airbrush the color on.

You can use royal, but its a pain. You can use buttercream, but is messy and sometimes still leaves bumps. Trust me, straight chocolate or ganache are the best, and the tastiest! :)