When I suggested the Poles, I was thinking of the novel The Teutonic Knights, by Henryk Sienkiewicz. The Teutonic Knights are demonized like nothing else and it's soaked through and through with projected Polish nationalism, but it's a very good story. Someone's translating it online at http://theteutonicknights.blogspot.com/
I know the Poles wouldn't make sense geographically or even temporally in the context of Livonia, but thematically I think they would fit right in.
Alternatively, could you simply port the Knight and Man-at-arms mercenaries right into the Rhodok troop tree, a la the later Polish-Lithuanian Commonwealth? The only other time we see these troops is in tournaments or under the player's command.
Finally, is there any reason a particular coat of arms should be restricted to one faction? I know there were trends in different countries, but azure, a bend or is the same no matter where you are.