We need some more data in order to properly diagnose this issue. It may or may not be real trouble. After 8 months I doubt the cycle is "stuck" but more likely something else is going on.
First, have you tested your tap water for ammonia? Also you should test it for nitrite and nitrate just to know, though from what you have written i doubt these two are present. But... won't hurt to test it for all three.
Second, what is the pH in the aquarium? This is significant with ammonia. I am assuming it is acidic (below 7), as ammonia at 8 ppm with a basic pH (above 7) would mean dead fish by now, assuming the test is accurate. Ammonia at any level above zero will have long-term effects, and above 1 your fish cannot survive for long and would certainly be showing signs of this.
And which test are you using?
This is not directly related yet, but I must comment on the fish load. This 55g tank will not support all these fish as they mature. Are you planning on a much larger tank, by which I mean a 6-foot or long? You can read on most of these species in our profiles, second tab from the left in the blue bar. Maximum fish sizes and minimum tank sizes are included for each species.