Well, if the refugium is going to remove nutrients and starve the main tank, I wouldn't recommend putting crabs or snails in the refugium. Let the refugium grow some macros and other algae and get 'ugly'. No sense in adding the refuge and it just being an extension of the main show tank.
I have used Mineral Mud (1/3rd cost of Miracle Mud) with sugar sand in a 10G refugium and it worked well. The Mineral Mud basically seeded my refugium with bugs and worms, and the macro algae goes nuts once it roots.
One last recommendation - if you are going to grow a macro that has a high growth rate, make sure you have easy access into the refugium. You will need to prune the marco algae on a regular basis. Have fun.
Ryland