My mangrove is currently in freshwater conditions - for the most part, they're very tolerant plants that will get on fine so long as they have enough nutrients to feed on.
It is possible I suppose, provided that there is sufficient space, light and waste coming from the fish. It also depends on what kind of mangrove you get. I'm not too sure on the species, but I know that some kinds of mangrove don't mind being mostly submerged, but others need to be close enough to the surface so that they can expose their roots to atmospheric oxygen. You need to do some research (ie: Google).
Some nurseries sell mangroves, sometimes you can find magrove seeds / pods washed up along certain beaches. I don't live anywhere near you so I can't help you there.
I've never heard of people 'acclimatising' mangroves... I moved mine straght from a brackish setup (SG 1.006) to freshwater with no problems.