I've kept mollies in my refugium twice in an attempt for them to clean up the algae growing on my macros. As for acclimation, just a few hours in a drip bucket in enough. They will have diarrhea, similar to travelers diarrhea so feed them well other wise they will loose a lot of weight within days and start to flounder. Do not get any males, they will aggressively chase females around till they weaken and die. Get at least 6 females, other wise the biggest female will give chase to the weaker ones till they die. Use only the sailfin, black, solid white or Dalmatian mollies, anyone with a red, gold tint would be hybrids of freshwater species from central.south America.
Now as for usefulness. My goal of them cleaning my macros was successful. they grazed at diatoms, cyano and everything else eating everything up, however, there is not enough calories to support them and they would starve while eating everything up, which forced me to feed them lots and lots of flake food, which caused more algae growth then they were worth. When I tried to cut down on feeding, they would again start to waste away on just algae. They could be grazing constantly, I believe their intestines are too short to be herbivores. Also, they may not be eating all the algae, but simply biting and spitting, like a dumb goldfish. As a comparison, I can feed a small damsel 1 flake a day and it would be fine. A mollie would need to eat at least 10x more.