Gymnogeophagus are the ones that require the cool down period ... regular Geophagus do not.
Yes, pellegrini max out at about 6"-7" or so and live in about 76-78F water ... but you won't find any. So far as anyone can tell, no pellegrini have been brought into the states so far. The fish sold as them are either crassilabris or steindachneri, usually steindachneri. There are no commerical collections from the part of Colombia they are found in (indeed, their natural range is rivers that go right through the coke plantations). A couple of crazy germans went down and braved the drug dealers and brought some back, but unless they come from germany and are quite expensive, you'll be getting one of the other two species.
Keep in mind these 3 species are not true 'Geophagus' and are quite a bit more pushy (the males at least) than you read for normal geo's.