1) Is there ONE supplement which I can use which includes most of what the
plants needs?

Dave has already mentioned TMG (Tropica Master Grow) as a good source for
trace elements. You will need a potassium supplement as well. I found
potassium chloride (KCl), or muriate of potash, at Home Depot.

2) Also, I am kind of mad at myself for not researching enough on the
gravel. From what I read I should have used Flourite. I'm thinking of
replacing the gravel with it it, or at least mixing some in.

Gravel will work fine for almost all plants. Smaller is better. I was able
to get this using plain gravel (and fish mulm)
http://personal.bna.bellsouth.net/j/d/jdjohn/recent.htm
Here's another example from a different Dave
http://www.okcomputerworks.com/aquarium.html

Jody