Most of the common livebearers in the FW trade can live in fresh for a while (as Witt said, natural forms do better than captive breeds). As for mollies, Black mollies do best in water with a little salt but not quite brackish. As for Sailfin mollies, they seem to do alright in any water. I catch them from salt marshes in full strength sea water down on the Gulf. You can keep and breed them in saltwater, thus making them the best choice for live feeders that you can raise yourself. FW fish that have been acclimated to salt are not as good. The fish needs to be raised in SW and eating marine foods in order to achieve maximum nutrition for our marine fish.
Scats and monos are probably the best bet for a "FW" fish that can be kept in a reef. I have been seeing alot of "FW" puffers kept in reefs lately (curiously after an online mag featured a pic with one). This is ok for some species for awhile, but most will not live their full lifespans like that. Also, beware that some puffers are entirely FW and will die if you try this, so do your research. There are actualyl quite alot of fish that can make these transitions, but they are not commonly available in the trade.
Saltwater Frogs