Okay, the GH should not be the other possibility.
Yes, the cycling process should have started with fish in the tank for over a week but that process takes time, usually at least 6 -8 weeks plus to complete. During the process, the ammonia and nitrite levels become toxic to the fish which means doing routine water changes to keep these levels at " acceptable" levels to keep the fish alive. This is going to be a long process of many months before your tank will be fully cycled and able to be fully stocked. Mollies and Platies are generally very hardy fish which is why they are commonly used to do " fish in " cycling. You will know when your cycling process has completed when you see 0 ammonia and 0 nitrites and a rising nitrate level. The nitrates are the end result of the cycling process. This is what it looks like in graph form:
View attachment 390181
You'll notice that the beginning and the end of the cycling process looks exactly the same with the exception of the presence of nitrates at the end.
I know this hasn't really answered your question about why only the Molly and Platy survive but there is more information you need to know before adding other types of fish to your tank.
What the end of the cycling process means is that there are enough nitrifying microbes present to handle the ammonia load present at that time. That means that if you have 1 fish in the tank, at the end of the cycling process there are enough microbes for the amount of ammonia that 1 fish or similar fish produces. The same applies if you use 10 fish or 100 fish or more fish. Your tank will be cycled for the amount of fish present at the end of the cycle. It doesn't mean that if you used 1 fish to cycle the tank that at the end you can add 10 more and not face problems. The biological filter bed is actually a living " organism" that grows and shrinks to the amount of ammonia being produced. In your case, you could cycle your aquarium with just the existing fish you already have or add more of them to help build a larger microbe bed ( a.k.a. Biological filter). If you do want to add more fish, you need to do this before the ammonia level starts to rise. Products like PRIME or other similar products do not remove ammonia so they are not the answer for an uncycled tank. You would need to do water changes to dilute the amount of ammonia present. Once your ammonia level starts to rise, you are best off not adding any more fish until the whole process finishes. Patience is the key to a successful aquarium.
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Once your tank has cycled, we can discuss fish selection in more details.
Hope this helps.