Okay, so I've had a look at the file and opened it up in the VEd to experiment around with it. Let's start with the armour. You can't combine clothing and armour together in a slot like that, because only one item can appear at a time per sprite. So I imagine in game you would have seen some guys with trousers and no cuirass, and others with a cuirass and no legs. So to combine the trousers and cuirass you have created, you need to create a new definition with those in the correct slots. I've done that for you here, using the "tunic" template in the VEd:
Code:
<VARIANT_MESH>
<SLOT name="tunics" >
<VARIANT_MESH model="variantmeshes\_variantmodels\man\trousers/celtic_cheque_trousers_01.rigid_model_v2" >
<SLOT name="armour" >
<VARIANT_MESH model="variantmeshes\_variantmodels\man\armour/hellenic_bronze_cuirass_a.rigid_model_v2" />
<VARIANT_MESH model="variantmeshes\_variantmodels\man\armour/hellenic_bronze_cuirass_b.rigid_model_v2" />
</SLOT>
</VARIANT_MESH>
<VARIANT_MESH model="variantmeshes\_variantmodels\man\trousers/celtic_tartan_trousers.rigid_model_v2" >
<SLOT name="armour" >
<VARIANT_MESH model="variantmeshes\_variantmodels\man\armour/hellenic_bronze_cuirass_a.rigid_model_v2" />
<VARIANT_MESH model="variantmeshes\_variantmodels\man\armour/hellenic_bronze_cuirass_b.rigid_model_v2" />
</SLOT>
</VARIANT_MESH>
</SLOT>
</VARIANT_MESH>
If you create a new VariantMeshDefinition file, paste that in and save it to the "VariantMeshes/_VariantModels/man/armour" folder that PFM accesses then you just need to reference the new file. The easiest way to create that file structure is to open up the "models_rome2" .pack file, expand "variantmeshes", "_variantmodels", "man" and finally "armour", then right click on any of the files you see and select "Extract -> Extract selected". PFM will automatically create the file structure you need for the above definition file.
So anyway, save that code into the "armour" folder. I saved it as "galatian_royalguard_armour". Then in your main unit mesh file reference it as follows:
Code:
<SLOT name="clothing" >
<VARIANT_MESH_REFERENCE definition="VariantMeshes/_VariantModels/man/armour/galatian_royalguard_armour.VariantMeshDefinition" />
</SLOT>
And then save your unit mesh file to the "VariantMeshes/VariantMeshDefinition" folder. Your unit will now have trousers and cuirass at the same time and will show variations between the combinations.
Second and more difficult to resolve is the arms problem. The trouble is that almost every barbarian skin model has problems with clipping through armour like the Hellenic cuirass. Essentially, you'll see a lot of nipples poking through xD
The particular issue you have is that all the different skin models you referenced only have heads and hands, but nothing else. The clue should have been in the file you referenced - "barb_headhands_bearded". This file contains references to five skin models which each have a lightly bearded head and a pair of hands. Basically, they are meant to be used with a model that has sleeves, like a lot of barbarian units, unlike a lot of Roman/Greek models with generally have very short sleeves.
I've only found one skin pack with arms that has no nipple-clipping issues, so for now I would recommend changing the "beards" slot in your unit mesh file to:
Code:
<SLOT name="beards" >
<VARIANT_MESH model="variantmeshes\_variantmodels\man\skin/barb_base_headarms.rigid_model_v2" />
</SLOT>
Obviously that means for the moment all of your guys will have the same base face model, although the variety of beards you have included should hide that from all but the most detailed inspections! Just play about in the VEd and try to find a model that works is the best advice I can give, unless you want to try and edit some of the models themselves and then try!
Hope that helps!