Good idea for the Leukanoi (forgot about them last time :P), but the reformed heavy cavalry imo it's too early, could accept them as wealthy Uolkai and other danubian Keltoi though, but such addition, should cut down the Brihentin...
The "local" assistance was from recently settled tribes in the Alps, kinsmen to eastern germanics; they would have nobility fighting as heavy infantry rather than cavalry...

The bulk of the cavalry should come from Bohemia and the Danube basin, but shouldn't be too wealthy: the Bouiroi did repel the Cimbri and partially absorbed them (employing them against the Skordiskoi)...
And from whence was this idea, that "reformed" heavy cavalry (a misleading term in itself) are not suitable to this context, spawned? It's notable that the Jutland Peninsula was one of the most Celticized parts of all of North Germania and that cavalry traditions were quite strong there.

How the heck would tribes in the Alps be kin to Eastern Germanic peoples? At this time the Eastern Germanic peoples lived in what is now Scandinavia and on the island of Gotland. Besides, any proper kin to Eastern Germanic folk would have their nobility armed as mainly heavy horse, as this was one of the famed traditions of the Goths. But since they aren't east Germanic, your point may have some validity.