The 500 rule is a quick way of determining the longest exposure time, for a given focal length, before the stars trail too much in the final image.
It works like this: you take 500, divide it by the focal length of the lens/telescope you intend to use and that will be the longest advisable exposure. For example, for a 50mm focal length lens, 500/50 = 10s. For a 250mm focal length lens, 500/250 = 2s.
A Nextar 8SE has a focal length of 2000mm. So 500/2000 = 0.25s! With a 2x Barlow it's even worse, since the focal length will become 4000mm: 500/4000 = 0.125s.
It's not really a rule, but more of a guideline. From my experience, when I was using my 50mm lens for some untracked shots, I noticed I needed to stay around 7-8s, before stars trailed too much for my likings (so, the 500 would actually become 350-400, in my case = even lower than the "rule").
Rule or no rule, I doubt you'll ever reach 15s with a 8SE. You'll likely start seeing star trails well before 1s of exposure, if the rule applies.