I wouldn't use the florist foam, it degenerates in the water and makes a mess. You could use larger rocks to maker an interesting aquascape or piles of rocks, just remember you'll have to clean around them when you clean the tank.
As far as I know, the indian glass fish are happiest and healthiest in brackish water. Corycats are sensitive to salt, so they can not be in a tank with salt in the water. Take a look at pristella tetras and blind cave tetras (your son might find these fascinating as they have no eyes!).