I wouldn't worry about a phospahte remover unless you are sure you have a huge amount of phosphate in your water. Waste of money. Have your water tested if you don't already have the test kits to see what your full parameters are first.
My Rhino got to 7 inches. That was the reason I bought him, smaller, so he wouldn't be one of those awesome looking huge suckers on MFK, lol. I think they get up to 10-12 inches if I recall correctly.
Actually my new tank is not setup yet. It will be installed in a wall and won't get any direct sunlight. I can clean the glass but am worried how the holey rock will turn out once it's covered with algae.