Just curious, but have you considered that the API test might be measuring total ammonia rather than free ammonia? When you use products that change the ammonia (harmful in small quantities to your fish) to ammonium (not harmful to fish), then the API test will measure both of them together, so you get a false positive reading in high quantities of ammonia. There are other types of tests out there that will give you two separate reading, one for "Free" ammonia (harmful) and the other for total ammonia. Maybe Hagen is giving you the Free ammonia reading.
There is also two other points I want to ask about, first there was a point at which you changed how you were adding the conditioners etc to your tank. You said you had been adding them after the full water change, which means there was probably plenty of time for the chlorine to kill your bacteria. Possibly your tank was not starting the cycling process, because of this and to much gravel vac action (I vac 1/4 to 1/2 the tank every two weeks) rather than just water changes.
The second point I want to put out there is that in order for your tank to cycle it has to have ammonia, and so if you don't allow some to build how can you expect the bacteria to grow in response? That is one of the biggest reasons you use cheap fish to cycle tanks. Most fish stores will take those fish back, probably not give you any money back, but they will happily resell unwanted fish. I guess what I am saying is to relax, watch the fish, make sure they aren't lethargic or gasping at the surface, believe me you will notice when they are having issues, but at this point in your cycling you shouldn't be doing such large water changes or so often (unless you notice your fish having difficulties).
The last thing I want to say is that if you have stuff growing on the glass of your tank your tank is well on it's way to cycling. It is an extremely good sign, but your tank is probably not fully cycling. This will eventually settle down, but for a while it will feel like an invasion from mars has taken place with brownish goo all over. Just remember that it won't last forever, and the best thing you can do is to allow it to grow on the back wall of the tank behind stuff. The fish will eat it, and it will help your tank in the end.
This is my humble opinion, but I do hope it helps. BTW, that's a pretty small load on such a big tank, I don't think adding the extra few fish will matter much except to slowly bring your levels further up. The pH will also lower when you put wood and other things into your tank, so that is normal also.
