I had a couple of Molly's given to me awhile ago and they are great in a reef tank..I have balloon mollys and no problem. They are algea grazers as well.
Molly's adapt very well into marine aquariums,just convert slowly if they exist in a fresh water system.
They are great for a cycling fish as well..They reproduce very well,so a little live food for the rest of our fish can be a little treat.(unfortunatly for the ones that can't hide good enough.
A few Molly's are great,although a whole tank full of them might be alittle much.