I tried to put one in my tank to help cycle initially but soon realized they aren't great swimmer and was soon stuck on the mp40

Why stress a fish?
 

Excuse me but I thought mollies are freshwater fish that like a little salt in the water.

Why you would guys unnecessarily put a fish through the stress of living in an abnormal environment when there is such a variety of small saltwater fish to choose from?

i have one that lives in an area of my sump. healthy and happy. prefers flakes over algea though.
if you have pairs they will provide live food for your tank.

Oh Oh, here comes the molly police. I did my homework before I put him in. They're pefectly fine in fresh or salt.

From everything I've read about Brackish fish is they are perfectly fine in both environments and naturally move back and forth between the salt and fresh threshold ...?

Correct.

And while they will thrive in a saltwater environment, it's actually detrimental to keep them in a completely freshwater environment - which most people do. ;)

I did this on first tank and the mollies thrived so well that they spawned twice before I sold them off.

Not only will they thrive in saltwater (which they do in the wild as adults), they're an excellent source of live food. I plan to keep a pair or two in my sump when I finish my upgrade.