The problem is that if you are start addin conceptual units, it'll be hard to stop. Should the Romans be given cataphracts, should every faction be able to recruit elephants, should all Hellenes get hysteroi pezhetairoi, and so on. There is evidence that the Gauls tried to create professional, well-equipped infantry. There is no evidence that they did the same for their cavalry and, if Ceasar is to be believed, what cavalry they had in his age performed rather dismally in battle. The latter suggests that they either weren't very good or very heavy.

Lastly, I'd argue that professional cavalry would not outperform the Brihentin. Warrior aristocracies tend to be quite well-trained: it's after all their purpose and there was no shortage of war in Gaul. In fact, heavy cavalrymen tended to consist of nobles through the ages because these would have been familiar with horses from a young age.

I cannot comment on the armoured spearmen, although I guess it would be possible. However, don't Neitos perform pretty well against cavalry in any case?