We had somwhere here topic about GA. As far as i remember there is little economic issue of having GA and airliners in same system. Discussion ended with saying that we would need stand alone separate flight management system for GA fleet (and partially for vintage or heli also).
Other part of Your post... Request. We have 737, 738 and 737C in fleet, 600 and 900 versions do not differ much from those. You may select flight made by one of those 737's and fly it with Your favorite 737 model, such as 732 or 739 or even 737BBJ.
We have currently quite big number of airplane types, and some of them are almost no used at all (around or under 100 flights per year). To be accurate we currently have more airplanes than pilots in roster (around 1,64 airplane per pilot in 2011). Further increasing airplane number/type will make only worse stats of each airplane. BUT we are adding some aircrafts to diffrent hubs based on requests, such as 777-300 campaign or recent e190 addition in LGAV hub.
Last year was also Concorde tested, even got few liveries ready and some special charts for flying. We got also Boeing 737-700C, A380 and CRJ-200 added recently.
