Unfortunately you are a victim of poor advice from the shop. You are now doing something called a fish-in cycle and it's going to involve some hard work for you for several weeks.
Ignore the pH level and nitrAte for now, they are not important. You need to concentrate on ammonia and nitrIte.
Fish excrete ammonia, it's the fish version of urine. But ammonia is toxic to them, it burns their skin and gills making it harder for the gills to absorb oxygen. In a mature tank, there are bacteria in the filter that eat this ammonia and turn it into nitrite. This is also toxic - it sticks to the fish's blood cells blocking oxygen from sticking on the cells. In a mature filter, another colony of bacteria uses nitrite as food and turns it into nitrate, which is much less toxic and only dangerous at high levels.
The problem is that there are no bacteria in a new filter; these bacteria are very slow growing and it takes several weeks to grow enough of them.
When you first put fish in the tank, they started excreting ammonia straight away, and this built up in the water as there were no bacteria to eat it. The nitrite level would have been zero as there was none being made because you didn't have any ammonia eating bacteria. Slowly, the very few ammonia eaters in the tap water you put in the tank will multiply. It will take weeks for enough of them to grow to eat all the ammonia continually being made by the fish. As the ammonia eaters multiply, they will make nitrite and this too will build up until enough nitrite eating bacteria have grown to eat it all.
You need to test the tank water twice a day for ammonia and nitrite, and any time you see a reading other than zero for either or both, you need to do a water change. I did say it was going to take some hard work! Depending how long ago you got fish, if it's only a few days you won't see any nitrite yet. But keep testing for it so you catch it when it does appear.
You need to change enough water to stop the level of either ammonia or nitrite getting higher than 0.25. You can change 90% of the water if that's what it needs, but with big water change get the temperature of the new water to the same as the tank water.
Here is a link giving more details on what you need to do. You can get through this, although it will sound a bit daunting.
Feed the fish once every other day or even once every three days. Don't give them too much food, only what they can eat in 1 minute. Less food mean less ammonia going into the water. Don't worry, they won't die of starvation. In the wild fish don't find food every day.
Now to the mollies. It is interesting that you are having problems with the mollies rather than the much more delicate rams. Though as they are balloon mollies, they have been quite inbred to get that shape.
What is your pH and is your water soft or hard - does you kettle or shower head fur up, that's hard water. Mollies (and guppies) like hard water with a high pH; rams like very soft water and a low pH. Mollies more than guppies can suffer if the water isn't hard enough for them. Since your rams seem to be OK, I suspect you have soft water and a low pH?
What kind of fish was Bob, was he a molly?