In using the Protocol on extraterrestrials, is starting with the oldest preferable? Is it advisable to do all three extraterrestrial interloper species equally, or is one group best to start with? Do some need to be done in smaller groups than others?

The oldest are usually among the most powerful. They are of equal need to work on. All have their roles in the Extraterrestrial Alliance and their liabilities for humanity. So all are in need of much work. You need not favor one species versus another or expect a greater burden, so you can treat them similarly.