How long have you had the fish? How did you acclimate it? You said you "changed your salt level". Did you do this gradually or a big change at once? Post your other water tests when you check them.
Phytoplankton is microscopic floating plant life, it's used to feed corals and other inverts. I'm not sure how to culture it. I just buy it in the bottles. If you run a search on this site you should be able to find info on culturing it. I remember coming across a few threads on this subject.
Just let the cycle run it's course untill the ammonia and nitrite read zero. You will still have nitrates. Then you can start doing water changes to get the nitrates down.
How big are the tangs? I'd say they'll probably need a bigger tank eventually. I would try to get the nitrates down before you add coral (as close to 0 as you can). What kind of lights do you have?
A smaller one would be fine in a 55 for a while IMO. I would plan on upgrading eventually though. I agree with leftyblite about their personallity. They are fun fish. I've fed mine out of my hands before but don't on a regular basis. I don't want him to start associating my hand in the tank with...