There are way too many South American cichlids that vary in size (everything from 10 in a 100 gallon, to barely two surviving fighting for territory). You need to be more specific in what you are interested in keeping, at least the general size.
South americans can range from the 3" max. dwarf Appistos to 2-3 foot long peacock bass. was there a specific type of south american that you are particularly interested in?