What's your ulitmate stocking plan? Your rock may have had enuf bacteria already present to combat/consume ammonia. A whole shrimp had no effect? How long was it in there?
Also, I'd be leaning towards questioning the test kit also as another possible culprit on this. I'm asking about your ultimate fish list because it may be something to consider adding them in stages and watching the ammonia while you do it. For instance, if you wanted clowns, being hardy fish, might go ahead and add them/a pair. Watch the ammonia for a week and see if it goes up. If not, then add another or two fish. Having said that, if you haven't a cycled tank, you're gonna be in for constant water changes if you add fish. Not a bad thing, but stressfull on them if the ammonina gets up too high.
Maybe get a 20g QT tank set up and have a place to remove/relieve ammonia stress if it gets out of hand in the main.
Oh yeah, stay completely away from coral for a bit. Gotta get your tank stable - or at least make sure it is as stable as it seems. You may have lucked out with the rock.
Only one fish in there, right? Not enuf to test the 90g for an ammonia absorbtion (sp?) test. The cleaner/pistol shrimp won't add anything.
Hard to say what your tank is doing, but just go slow, do your hardier fish at the beginnning (clowns. chromis, gobies etc) and you'll be OK IMO