The only restrication we have from that is that we only draw dragon characters when the dragon expansion is played, as they tend to be more powerful than the others (I know it's not true for all of them but we handle them as a group).
That's kind of odd, as most of the characters are weaker when not used in combination with the Dragon expansion. Abilities that are directly related to dragons or breath attacks see much less light of day when not used in combination with the Dragon expansion.