Confused

When in the local pet shop, they have various types of cichlids, the one's
that are in the brackish water have more color to them and live in what I
think to be coral, then they have the regular freshwater fish, and finally
they have the freshwater cichlids that are in my tank:
Firemouths, Red Devils, Jaguars, Rusties, Convincts, Jack Dempsy, Green
Terrors, Jewel Fish etc..... Upon a closer look at the brackish tanks in
the store, I noticed that they had a few Rusties in there! I spoke with a
sales clerk and he said that most of the freshwater cichlids can adjust to
brackish conditions.

So now I"m confused, ( I thought that this was a freshwater cichlid forum
but we talk about fish that are not really freshwater quite often) I can't
figure out what conditions my fish need, I wonder if keeping them in
freshwater is incorrect and that perhpas I need to change to a brackish
tank. I'm trying to figure out whether there is a book that I can purchase
to educate myself on the differences in Cichlids, I would like to understand
their background and where they come from and was wondering if there is a
book like that on the market, or perhaps a website, an easy to understand
manual or something or if anyone with experince that is willing to assist me
would be much appreciated.

Mariana