It makes sense. While Lavos didn't exactly merge with Queen Zeal, he still managed to influence her into becoming the dark harbinger of death in 12,000 B.C., so it's not far-fetched to believe that Lavos did the same thing to Schala before they became the Dream Devourer. Each member of the family has their own "dark side" which was more or less the result of Lavos's presence, whether direct or indirect.
What I'd like to know is, the Lavos that fused with Schala...was that a new Lavos spawn or the same Lavos that Magus and co. destroyed?