We're assuming that you want to write an alien psychology that doesn't match up exactly to the human mind, so we're going to stick closer to the Hard end of the Mohs Scale of Sci Fi Hardness. It's fine to veer off from it, of course, and your only issue with soft sci-fi may be that you dislike Rubber Forehead Aliens and there are few writers whose stories have Soft physics and Hard aliens, but if your story is Soft you might want to make it clear to the reader beforehand that your story will have Babyeaters, not Klingons.