You can use any models you choose, although it may be a bit awkward if you use larger, say, ogre sized monsters. While the various soldier types have different stats, their "what you see is what you get" aspect is based upon their equipment.
For example:
Dagger = Thief
Hand Weapon = Thug
Greatweapon/Heavy Armor = Templar
Hand Weapon/Shield/Heavy Armor = Knight
Hand Weapon/Bow = Ranger
Etc.
Therefore, a goblin with a club and no armor would be the equivalent of a Thug, while an Orc with heavy armor and a greataxe would be a Templar. You could possibly say that goblins are Thieves since they are weaker melee combatants, and so on, but it may make things more difficult for your opponent.