We already have instances of glitchy "hybrids" appearing sometimes when simmers make two different occults have a child. It's really buggy; like with a mermaid- vampire hybrid, they won't be able to change their dark/mermaid form. They also won't be able to level up as a vampire. This bug has been reported numerous times, yet the sims team hasn't gotten around to solving it. So if the developers aren't gonna fix it, they might as well go ahead and create legitimate hybrids. It's seriously aggravating how the sims team hasn't fixed a lot of issues that have been around for so long.