The AI seems really confused by the regional and faction recruitment. Since the Romans can only recruit Roman units in Italia, they have to recruit other types of units in the places they conquer. Since the AI seems to build as many units as they can, especially in border territories, they get lots of non Roman armies. The only way to help them make Romans would be to lower the cost of Roman units and that would make much since for both human Romans and historic realism. Or you could go the vanilla route and have them trait Romans everywhere, which is way ahistoric.

The Neitos are late period Gallic troops. The Gauls have to wait for a reform or two in order to train them. But since building conditionals don't work for unit recruitment, the Romans can either get them at any time in Gallic provinces or never get them. They make a good "allied" unit to fight with Romans, or even a "Gallic Legionary".