Same, and I never used any mod. I have the exact same problem and now I see I'm not the only one.
My mermaid and vampire had triplets (one who is only a merman, and two who are vampire-merman hybrids ). The hybrids have all the mermaid related powers, need blood to survive, fear the sun and can use some vampire powers, but they can't evolve and get other powers and I can't edit their dark form or their merman form. So they are basically normal mermen with some vampire powers and can't swim during the day because of the sun burning them.
I tried with a mermaid and an alien: they had an alien-mermaid hybrid who has all the alien and mermaid powers. I can edit her alien disguise but this disguise is also used as her mermaid form in water (she doesn't have a tail and swims just like a normal human sim). So she is basically an alien with some mermaid powers.
I think the Island Living pack somehow allows the creation of hybrids (mermaid + alien/or vampire) but the different forms (dark form, mermaid form and alien disguise) are not compatible.
PS: I'm sorry for any mistake, I'm not a native english speaker. I hope my message can be of any use.
