Do you have eggcrate beneath the sand of your tank? If you use 'one big tall rock' the fish will make their own hiding spots by burrowing under it, potentially making the rock shift leading to a cracked glass or catastrophic failure.
I would clear the sand from where you want the big rock, then place the rock and pile the sand back around it (to prevent shifting), an then use some of those smaller rocks to create nooks and crannies against the big one to provide the hiding spots that the Africans crave.