Writer Carlos Cisco has indicated that they conceptualized the Breen's solid form as an adaptation that they developed over time, which would track with your idea.
And "The Chase" certainly left some wiggle room for there to be species that were untouched by the Progenitors:
Our scientists seeded the primordial oceans of many worlds, where life was in its infancy. The seed codes directed your evolution toward a physical form resembling ours. This body you see before you, which is, of course, shaped as yours is shaped, for you are the end result.