Ummm When Super Saiyan was first mentioned in DBZ (If i recall) they show the image of a super saiyan being an oozaru with some sort of large golden hair, then showing himself being some giant thing on a small 'moon/planet' and blowing himself up ...
Im not an exact expert on the whole Dragon Ball -thing- so anything i can say im happy to be corrected about, but if it hasnt been 'hammered into stone' that Legendary was actually a state, it isnt - sure they may of been some mistakes on the idea but yeh, all we're doing is interpretting(sp) What hard proof we have, which is official or unofficial.
I always thought that (and in general before Buu what SSJ-SSJ2 was refered to in DBZ) as an extended form of Super Saiyan, then after buu (this is in the TV show in DBZ, i dont know nothing else) they started throwing numbers like super saiyan 3 and stuff. But u could then say that all forms of SSJ are just the persons trained level of such, so just to add more to the point Legendary SSJ is just a Broli's level that he can extend to, and with all other 'levels' of SSJ its the just the person controlling their power and being able to handle more power, anyway whatever didnt wanna go into that much extent... bleh