I fixed your problem,

this is my first post so I hope this works!

to get the pictish crossbowmen go to your Bi data folder then go to export_descr_buildings, then find the words building missiles. When you find that scroll down a little and you'll see recruit written a lot of times. at the bottom (below "western archer") type this,

recruit "celtic crossbowmen" 0 requires factions {celts}

I hope this works for you.