Black mollies are your best bet. They are better suited for the higher
ph and temps of a
SW tank. Best suggestion is get the tank set up and cycled before you do. Having
LR is just fine and will offer some hiding places and possibley grazing foods for them. Not to sure they are grazers though.
Believe it or not they can be transitioned in less than a day. Place them in a container appropriate for their size and the amount of water than can easily be removed from the
SW display tank. One
gal per fish should do but only fill about ¼ with
FW for the fish. QTing for parasitic problems is not a concern. The parasites will not survive the transition.
Slowly drip the
SW from the main tank into the container over several hours. Once full, empty ½ the water and discard. Keep doing that until the salinity of the main tank and the molly "pen" are the same. You may need to re-acclimate them for temp but after that, net and place in the display. Be sure to discard all the removed water so you do not alter the main tanks salinity. Then simpley replace with well aged new
SW.
Mollies in general are pretty messy fish so I would not recommend them for smaller tanks unless you like alot of maintenance
Cheers
Steve