I have a male Dwarf Gourami, a male and female Opaline Gourami, 2 silver dollar fish, 3 Austrailian Rainbow fish, 2 black mollies, 1 lyretail dalmation molly, 3 different types of platties, 8 various guppies, 5 ghost shrimp and one large pleco. They all are happily co-existing, the only nipping in my tank is by the guppies (well and the silver dollar fish, on a plant every now and then)
The Dwarf is about 2.5", one opaline is about 2" and the other is 3.5". It can be done, but like someone else mentioned, make sure you get a healthy one, and one that doesn't seem to be very aggressive in the tank at the store, we always watch the fish for a while.
ON a side note... we think our guppies have become ich detectors, our LFS has a couple of tanks that i think are infected with ich, whenever we get a fish from one of these tanks a couple of the guppies (tequila sunrise) will ALWAYS nip at their scales, but ONLY on the fish from those certain tanks at the store. Well, these fish 98% of the time end up having ich, we move them to a QT, treat them, let them recover and move them back to the main tank, and all is well. It could be a coincidence, but it's an awfully consistent one.:smile:
I'm sorry, I just noticed that you said 3cm long molly... I'm not sure, the only things this small in my tank are 2 guppies.... I hope this helps.