When I've run that adventure, I forced all my players to have human characters, using the ambrian background (which would be the same as for an alberetorian).
I filled them with little information about what Ambria was all about, just about the effects of war in Alberetor, and promises and stories about Ambria. This way, the first times they encountered a goblin or ogre or elf, etc was a special one: the players, having the usual "fantasy rpg" background where goblins and ogres where bad, and elves good, were as surprised as their characters would be. The same applies to undead (Mal-rogan being a speaking, thinking adversary). In this case, the metaknowledge of the players helped inmerse in their characters.
I need to add that, both times that I've GMed Symbaroum for a series of adventures, I've planned the campaign in the form of "chapters", where the players could change their characters at some points, thus allowing the introduction of other races as playable, but only once they were "unlocked" during gameplay by their humans from Alberetor.