Whilst swords, axes, clubs and other melee weapons of the sort may well have been mixed within a formation of soldiers, given that the celts and particularly the germans mastered the shield wall formation early on, I doubt you would see spears as a primary weapon mixed in with swords as a primary weapon. There is one exception, and that is when it is used as a tactic. For example one germanic practice was to have the front line of a formation harmed with long pike-esque spears whilst the ranks below were armed with a sword and shield.

The idea that a celtic tribe "would not recruit descrete units of spearmen, swordsmen and axemen, but instead call out the whole of the tribe's warrior who would be armed with a variety of arms and armor, rather than all armed the same", just lacks common sense. An army like that wouldn't last two minutes, and I would be rather suprised if some of the greatest cultures of western europe didn't learn the basic lesson of combined arms.

However, I'm still drawing a blank on a group of units that the OP could possibly be referring to. It would have to be a group, the unit of which represents the same region and same status of warrior, the only difference being what weapon they wielded. In all cases that I can think of, we have chosen a weapon most suitable for the unit, though with the obvious caveat that it would not have been the only type of weapon soldiers within the formation would wield. But the unit came first, not the weapon.

Foot