I've made a couple paludariums using styrofoam coated in a sand/epoxy mix.
The ones I made were glued in, so they would be really difficult to remove, but I'm toying with the idea of getting a big enough block of stryofoam that I can effectively wedge it in the tank, with just a couple dots of silicone to secure it/seal the edges. That way I think I should be able to pull it out (breaking the stryofoam around the silicone), and then just scrape off a couple silicone blobs. Haven't tried it though, so there are probably all sorts of unforeseen problems.
Also, if you do decide to remove it, you could try setting the tank on it's back, and using some solvent that dissolves stryofoam, pour that down the edges, etc. You'd just have to make sure it's not something that will affect the silicone, and keep it away from the rim.
Anyways, I really liked the results of a sand-epoxy mix. Real easy to use, gave a nice hard, rough, rock-like surface, and it was heavy enough to hold the styrofoam down (this depends a lot on your design, and volume-surface area ratios, etc.). If you used a more viscous epoxy resin, you could probably end up with a fairly mold-able sand/epoxy mix, and directly form your shapes that way.