Guess it depends on how you define what's a reef and what's a mangrove forest, as nature may not make a distinction.
You could do a mangrove forest and add various corals to it, as there's no real reason they can't go together, or you could section off part of the tank or have a connected tank dedicated to mangroves and seagrasses and so on and have a more traditional "mixed" reef in the other part.
Julian Sprung's pond for example has a huge (by hobbyist standards) mangrove tree in it and also has some high energy corals, clams, sponges, etc.
A lot of the tanks we have are totally unnatural mixes of animals that come from completely different environments. In the wilds, mangrove forest can often house various types of soft corals, sponges, "reef" fishes, urchins, etc. A "reef tank" could be as simple as rocks with some zoas and a few small fish. In that case just adding some mangroves would create a fairly natural and normal mix of livestock.