There beautifully! But the female Sirian looks too muck like an Asari and a Quarian with a little bit of Drell mixed in specifically noticeably in the male portrait. I think that The-Brade may need to rethink what these aliens should look like.
Maybe you should go back and talk to The-Brade about this. There is nothing wrong with being inspired by a favorite game or other media franchise but if these characters are going to be sold to the public that could be a problem.
#6 isn't that impractical if perhaps the Sirians had more aquatic ancestry, we are talking about life outside of Earth where conditions that life could spring up would blow our limited human minds out of the water.
6 is a big whole,.. easily to infect and for parasites. (maby some fish could hide inside there) This form of Scull is also very instable, ore if very thic bone, the head gets pretty heavy. It reduces the space of Brain inside, and i hope these Sirians are like duckbills and lay Eggs, because it could be very complicated to get out for the baby^^
But these are just some thaugts,.. i could be also wrong, .. i am no botanist