The thing about managers and employees of pet stores is they often have fairly little experience, and whatever experience they do have they throw out of the window. It's a sad fact that they're probably more interested in your money than the well-being of your fish.
Now don't be so hard on yourself, it's happened to ALL of us when we came into this hobby. The best you can do as a newbie is if you see a fish you like in your LFS, go home and read up on a few different websites, post questions on forums, whatever you need to do to make sure it's suitable for your tank and the current set-up you want. It always works to go into a shop with a game plan so that the clerk can't palm you off with whatever he/she "think" is good for your tank.
Now for the goooooood news: you have a very nice sized tank. If you wanted to start again it wouldn't be that difficult, if you wanted to you could take your mollies back to the shop and swap them for something else, unless of course you wanted to go down the brackish route (which I get the impression you don't). There are plenty of fish that would work well in that type of set-up.
Also, how long has your tank been set up? Chances are it's got a fairly good colony of beneficial bacteria in the filter (btw, if you ever need to clean this, just squeeze out the excess gunk into a bucket of TANK WATER and then you avoid killing this bacteria). Cycling is just the build-up of colonies of beneficial bacteria that convert the toxic ammonia in fish waste to toxic nitrIte, and then to harmless nitrAte. It is very much worth buying an API FRESHWATER MASTER TEST KIT, to check the levels of these substances are in check. A good reading should be 0 ammonia, 0 nitrIte and between 10-20 nitrAte.
I hope this information helps, I have a habit of waffling! And also in future if your fish get ich, if you see it early enough, a bit of salt in the water (1tsp per gallon) and raising the temperature to around 28 celcius, plus adding extra aeration via air pumps should help this.