Well, it may be too late to answer this anyway, and I'm no expert on mangrove growing, but I can try (plus this thread is floating around the top of the forum cause no one's actually answering it).
I would think you'd only need 3-5 inches of substrate for mangroves that size, and maybe you could do with a bit less. Since they're not going to be growing too tall, this should support them enough. They do like to put out air roots to get enough oxygen for their root systems, so I think water depth might be more of a concern, but I doubt there should be much issue in a home sized aquarium.