We've seen the Inquisitor fighting a group of mages and templars fighting each other
They've told us of seeing even wild animals in the same location sometimes fighting each other
In the recent demos they fought avaars and demons, tevinter fighters and demons and mages
Good luck trying to find a perfect combo for such encounters, which shouldn't be rare unless of course enemies suddenly got a restriction order to not approach other enemies while the player is fighting them
Depends on what their weaknesses are.
Do Templars and Mages share elemental weaknesses?
Do wild animals?
Do demons and Mages?
Or do they share them across groups, like all Avvar barbarians, regardless of class, share the same weakness?
Are they fighting close together? 'Cause a good AOE attack will settle that one right quick.
Of course, what I'd do is make sure that my team had reliable, effective abilities that can be used in a variety of situations, with weapons with different elemental abilities to cover each other's weaknesses.
Going back to Pokémon team-building 101. Which is good, 'cause my skills there are rusty.