The wraiths of Dakala are clearly not the same species as the founders. For one, the wraiths have a completely different culture from the founders. They are indigenous to Dakala and appear to use no technology and have no ability or desire for space travel. They also don't have the great link or the natural tendency to revert to any type of liquid state. In fact, their natural form resembles a sort of slug:
versus:
Similarly, the founders don't have telepathic abilities (they can communicate nonverbally through the link, but they can't read others' thoughts directly), and they live very differently from the wraiths, whom have a much more nonhumanoid/alien intelligence than the founders (despite all their rhetoric about how they're not humanoids and the drop becomes the ocean, blah blah blah...). In fact, it's not even clear that the wraith which contacted Archer actually spoke to him in audible words or if it were just telepathic.