Are you using the encounter editor to change the units in that fight? Or are you using the recruitable character Nightmare Module to change the units that join your party? I'm not 100% sure but I think if you do the latter and alter the recruitable units but don't change the units in that encounter, it'll play the tutorial out as normal but then replace the recruits with the ones you chose afterward.
EDIT: Never mind, it looks like the module I'm suggesting can't change their races at all, just their loadouts.
EDIT: Never mind, it looks like the module I'm suggesting can't change their races at all, just their loadouts.