I've got to say that I really disagree with the point about the clown loaches. If they get to be 12", you'll have a story to write. In hobbyist aquariums they are likely to stay under 8" tops, and more likely around 6".
I also would say that platies and mollies can survive happily in non-brackish water. Many breeders keep them in fresh water, many suppliers ship in fresh water, so if your lfs is responsible, they will keep those fish in non-brackish tanks.
In fact, if you buy them in such a store, and put them into a tank at home and add salt, they are likely to get stressed. And vice versa, if at the store they are in brackish tanks and you bring them home, you need to keep them in brackish water.
So the trick with platies and mollies is to give them what they are used to, and if you want these other fish with them that means non-brackish. Buy from a store that keeps them and gets them from non-brackish water, and you'll be fine with them.
BTW this is not going to be difficult to achieve. At least on this side of the pond, many many suppliers keep them in non-brackish tanks.
But all this said, you'll need a large tank and there are the compatibility issues re:t