Well, sadly this isn’t going to be in the official story of the Friendship Cup. Many people wants to see the Dazzlings with the Dark Stars. I never saw them as rivals; the only rivals of the mermaids are Mane 7 and the Muses. In this story the Dazzlings are only mentioned a moment by Flash Sentry. This is what would happen if Dazzlings will meet the Dark Stars.
A curious fact: In Spanish, my native language, sirens and mermaids have the same translation; which is SIRENA.