I have 14 mollies in my 180g. 4 black, 6 dalmatian, 2 balloon and 2 red sunset sailfins. They are excellent at clean-up - but here are some warnings from my experience:
1.) They are voracious eaters when multiples are in the tank. While they are not aggressive to reef fish, they do not have the same fear reef fish tend to have towards people. So they will push to the front of the line if you have other more timid or passive fish.
2.) They breed. They breed a lot. Like, always. They really should be called rabbit fish, because all they do is breed. My males have actually killed females due to exhaustion trying to breed. This behavior can be a distraction in the tank.
3.) They are great nuisance algae eaters! However, that algae gets processed. I consider these fish just about as dirty as tangs. Since they eat a ton, they poop a ton. Perhaps its just something with algae eating fish, but it seems excessive. My recommendation is to be certain you have some method of removal from the sandbed before putting them into the tank. A good cucumber or two should do it.
Just remember, there is a trade off with these fish. And with acclimation, you can simply toss them in. They can tolerate hyper salinity or hypo salinity at the drop of a dime. They are delta dwellers and in the wild are used to rapid shifts in salinity. Another good thing about them is their ability to adapt to higher flow in the tank. When they come from the store, they'll most likely be in a tank with low flow, but will quickly strengthen up to the flow in your reef (it's fun to watch actually.)