The bracae are in, with Iberian units at least. As for Celts and Germanics, it will depend on the historians. Can't say yes or no at this point.

The pilos helmets are widely used in the hellenistic units. Have you checked the stele #6 ? As for slingers not using hats, it's debatable.

As you may understand, all features in the models, including the facial, are defined by historians based in evidence, sources, etc. As you said, we may have bearded and beardless warriors due to the multi-object concept in MTW2, so we will exploit that feature as much as we can.