Do not add any ammonia to a tank with fish in it unless you are holding the net in your hand to remove all the fish that it kills instantly.
Ammonia is the stuff you are trying to get rid of by cycling a tank. the fish produce ammonia by eating the food they get and then eliminating their wastes. That is why the advice you got to hold back on feeding somewhat but don't let the fish starve. Ammonia is useful in an empty tank to simulate having fish in there, the bacteria won't know the difference.
Where you are right now means big and frequent water changes just to try to keep some of your fish alive. When you get a good technique going on water testing you may be able to do only daily water changes. Until you start reading safe levels I would do at least twice daily water changes of at least 25% of the water. The test kit will be easier to read when you are closer to reasonable levels of pollutants in the water. After you get that going, you can start to look around for a way to rehome some of the surplus fish. As others have said, you are not just overstocked but have incompatible fish.
A sumersible heater should be under the water, adjusting thingy and all. It will probably have a water line on the outer tube that should always be below the water line, even during water changes unless you unplug it.
I have used the Jungle Start Right product. It is a decent dechlorinator. Right now I am using the seachem Prime because although its slightly more expensive per bottle, it treats many times more water so its much cheaper to use. You use drops of it instead of capfuls for small quantities of water like 10 gallons. It takes a capful to treat 50 gallons instead. For me, 50 gallons is not an unusually large amount for a normal Saturday but I have more than one tank.