FERRIS, ITS ONLY PERSONAL PREFERANCE TO TAKE TIME IN FILLING THE TANK, AS HIS OLD TANK WOULD PROBABLY ONLY FILL A 1/4 OF THE LARGE TANK, IF THAT, it was more to prevent osmosis shock. his tank will more than likely have to re-cycle again so he will suffer the nitrites and nitrites anyway. its like adding a sink full of cycled water to a bath full of tap water. I feel by doing it slowly, he is only slowly altering the water parameters and giving the fish more of a chance to adjust. does this explain and sorry caps lock was on (not shouting)
I advised fiesta to add a trio of peacocks when he was at my home, as he will vouch for mine, they are in with full grown mbuna, and they are perfectly happy and in very good shape!!!
hiding the fluval behind the rock may be a problem, as that tank has large strengthening bars, and as you know the fluval pulls out from the bottom. because your filtration is not all that big you will probably have to clean it more, especialy as your mbuna grow. do you realy want to dismantle your rock every time you want to clean the filter?
I bet you are looking foreward to having to strip down your tank again to move it onto a stand?