It is best to use damsels to break in a new tank. If you are then going to add other aggressive fish, you can keep the damsels. If you want to keep shy or delicate fish, you should take the damsels back to the pet store once you and your tank are ready for more fish. Some damsels, such as the blue damsel and yellow tailed damsels, are not as aggressive as others, such as the three striped and domino damsels. In any case, damsels are certainly the best fish to start with.
Some people like to break in a tank with mollies which have been acclimated to salt water.
Although safer, you don't achieve much marine experience this way. Mollies are captive raised and bred. 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.