You don't want ram cichlids -- Mikrogeophagus ramirezi, at least -- in your 75 gallon tank. Rams need very soft, very warm water, and not all of your fish will appreciate that. Corydoras will boil at the 30 degrees C that rams need, and platies will simply keel over and die if faced with water with less than 5 degrees hardness, as you need for rams. In conditions that don't suit them, rams are notoriously disease-prone, and a lot of the fish sold are "juiced" with hormones and antibiotics anyway. So as soon as they leave the breeder, they start dying by inches...
By all means get Bolivian rams -- Mikrogeophagus altispinosus -- but a better waste of money I cannot name than buying generic rams for a community tank.
Cheers, Neale