The most common puffers found in the hobby are indeed brackish water fish, and would do very poorly in Rift Lake water, which is high in mineral content but very low in sodium chloride (which is what makes brackish water brackish). There is a species that is common in Lake Tanganyika, but it grows to over two feet long and is equipped with a beak that would allow it to fatally injure a cichlid almost at will. So no, the combination would not be a good idea. A small species tank would probably be your best way to go.
