serial murder is defined by the continuity between murders, whether those are the victim profile, the method of murder, the method of disposal of the bodies and/or the "signature" of the killer
spree killers are generally indiscriminate about their victims, choosing targets of opportunity rather than carefully chosen victims from a profile which may be obvious only to the killer himself.
"disorganized serial killers" is oxymoronic, as the care discrimination and organization of the serial killer's crime is paramount, while spree killers tend to choose targets randomly with little interest in how they fit into the killer's delusions. spree killers often target an entire class of persons sometimes arbitrarily, eg: everybody around this clock tower, everyone in this school, any black person i meet, etc...
then theres just plain multiple murderers, like tookie williams, who murdered for profit and sport, with no interest in the victim save that tookie had the advantage on them and thus was able to kill them with impugnity. anyone falling into tookie's power was as likely or not to be his next victim, but he had no internal motivation of delusional justification that would allow him to be classed as a spree killer, he was just an asshole.
psychology and psychiatry deal with each of these types of killers differently in profiling, as well as classification. your citation of the fbi's law enforcement perspective is irrelevant, and also disingenuous since pursuing a spree killer (like andrew cunanan) is just a manhunt, while a real dyed in the wool serial killer like richard ramirez can require years of painstaking investigation just to uncover the extent of his crimes, much less the identity of the killer.