A couple of things I've found is.
1. Identity Types
Where you configure the CfgVehicles class in the config.cpp for each unit, you can make an entry related to identityTypes - this is where you can tell the config the language spoken by the unit, what types of faces to use and even the types of sunglasses the units wear. It looks like this:
identityTypes[] = {"Language_EN","Head_USMC","LHD_glasses"};
You can also set it up so that the units use a specific type of sunglasses or face only and the config for SchnapsdroSel's Desert Merc units are a good example of how to do that.
2. Heads
I've also discovered you can include the head models from ArmA1 units (ie. in the .p3D file for the unit) in your ArmA2 addon as long as you don't add the new invisible head proxy used in the ArmA2 example unit. The only thing you need to be aware of to make this work is to make sure that any head gear that the unit is wearing (helmet, hat etc) needs to be part of the head selection in the model lods otherwise it won't show up in game.