While reading "Ancient and Medieval Siege Weapons" by Konstantin Nossov, I came across an interesting fact that may help to shed some light on why the five champions become the five companions once they rescue Alric. Here is an excerpt from the section on Alexander the Great:
"By the beginning of Alexander's rule, the core of the Macedonian army consisted of heavy cavalry (Companion Cavalry) and heavy infantry (Foot Companions). Among the later was an even more elite detachment of hypaspists ("shield-bearers"), which had probably been formed under Philip II (Alexander's father) but was first mentioned only during Alexander's era... Alexander also used hypaspists as well as other detachments of "companions" at sieges, sending them to the most dangerous places."
From both this account and the posted link, Alexander's Companions were elite warriors who were trusted with the most dangerous and important tasks and were fiercely loyal to their general or king. It seems likely that these Companions were the inspiration for calling the five champions companions once they had rescued Alric. After all, the five champions were not only responsible for rescuing Alric, but (the berserks at least) were also trusted with the missions of retrieving the Head from Muirthemne and carrying the Eblis stone to Rhi'Anon.