From what I just read they don't seem to grow that fast in the home aquarium. If you want to do a little reading this is what I just went through and it has convinced me to pick up a couple this afternoon from BRK. The only reference I could find about trimming was this:
Pruning red mangroves is a sensitive matter, though, and a rather moot point for most aquarists. Even under the best conditions, mangroves really are slow growing for most folks… taking three or more years to even get 2-3 feet tall. If you must trim your tree, be sure to delay any pruning until after the axial tip has branched. Damage to the growing tip before it has split can be fatal to young specimens.
Read the article it was very interesting and I think I'm going to do the inline setup in the bucket next to my tank so I can have a nice plant to enjoy as well. I have enough light put off from my MHs that I shouldn't have any trouble growing them next to the tank. I think I will start out with them on the PVC
stand described in the article in my sump until they start to grow a bit and then do the bucket or maybe another inline style setup.
Hope that helps.