From goldfish to Cichlids

So I want to switch from my comets to Cichlids, I had planned to upgrade my goldies to a 75 gal but want to do a Cichlid tank instead. so here is my questions:

What equipment do I need? (I had planned on getting a Marineland Emperor 400 for the tank)

I was interested in the Red Devils, Green Terrors and Convicts. Can I put these together or is it better to have only one species at a time in a tank?

I also wanted to do a planted tank, can I put java ferns and hornwort in it with the Cichlids? (its the only two I decided on for sure)

Will my 75gal tank be big enough for more then one pair? (I heard they were suppose to be put in pairs)

Or in other words, what can I stock in my 75 gallon tank together and do I need anything different from a goldfish tank?

This is what I planned for my goldfish tank:

2x Marineland Emperor 400 or 1 Marineland Penguin 350 with a external canister filter
Stock lights
Air pump w/ air stone
Black gravel/sand

Hehe I'm not, I've never had a planted tank so its not much of a heart lost.

So all I would need to add to the list is a heater? I know for Goldfish its good to have ALOT of filtering, are Cichlids the same?