Some people like to break in a tank with mollies which have been acclimated to salt water. This gives you the benefit of starting with inexpensive fish and get used to maintaining salinity and pH on not-so-sensitive fish. Although safer, you don't achieve much marine experience this way.
If you buy mollies for your saltwater tank, you can acclimate them by dripping saltwater into the bag over a period of 6-8 hours, removing some water when the bag gets too full. Slowly increasing the salinity gives the mollies time to get used to their new environment. You can keep the mollies in the tank after it cycles, but any aggressive fish with continually harass the passive mollies.