Yes I exaggerated the Senone sack, but they composed much of the army and they were not under the rule of a central Gallic authority (while coalitions sprang up there of course never was a unified Gallic kingdom). The Romans refer to the Caledoneans in the era we're talking about rather than calling them Picts, but in the absence of evidence indicating a migration I would say the Picts were descendants of the Caledoneans, and "Pict" is a suitable generic term. Other tribes that could be included to simulate the diversity of politics and culture could include the Belgae and Marcomannes.

P.S. nice to know people are particularly picky about their history around here