the reason u get the peasant unit card is because when the game cannot find a unit card for a specific unit it uses the default one, the grey peasant. To change this u need to unpack the xpack files, create a new unit card and place it in C:\Rome - Total War\Data\UI\UNITS\PARTHIA this folder with the appropriate title ie #EAST_HEAVY_INFANTRY or some such thing. i think this is wot u wanted to do sorry if its a bit vauge but im not hte best at explaining these sorts of things if u need more help maybe post and one of the more experienced guys can help u out.
cheers
oh and welcome to the forum's