Ich is a parasite and it's not alway present in your tank. Fish can get get infected if ich is introduced into the tank, usually by a fish carry it, water from a system that has it or from LR from a person who tank has ich. But if it's not in your tank the fish aren't going to get ich, regardless of how stressed out they are.
Whether a fish is stressed out or not has nothing to do with a fish getting ich, as healthly fish can and do get it. What stress does is usually make it so that a fish can't fight the ich off, and usually they will die from it unless treated. Also ammonia and ich have nothing to do with each other, if anything the high ammonia will usually kill the fish outright or the fish will get an infection from scratching against rocks, which is something that happens when you have high ammonia levels.
As for adding dead food, shrimp ex., you're adding additional sources of wastes, which will build up the bacteria in the tank. Once the tank is done cycling the bacteria will be very strong and can handle a big load. This way once you start to add things to the tank the bacteria can handle it and you wouldn't go through another cycle or have ammonia in the tank, as the bacteria tries to catch up. This is the case in a lot of tanks just starting out. If you're bacteria isn't strong enough, to handle what you put in the tank, things are going to die and you're going to end up looking at an empty tank.
In the mid 80's people used to use productst to boost the bacteria levels during cycling, I forget exactly what it was but I think it was called Ammonia Cloride, which you could get at a chemical ware house. Spykes(Dave) actually has a thread on it, as he was using it to help with the cycling of his tank.
