Ah, I've created a thread collecting these issues on AnswerHQ and asked the developers to reconsider the filters.
Yeah, Sulani Elementals are generated with Death by Old Age trait and the filter simply requires them to be an unplayed ghost Sulani Mana.
Putting on another ghost trait is kind of a solution. The thing is that when asking a Sulani Elemental to move in, they should lose their ghost status by having their Death by Old Age trait removed. However, as other ghost traits aren't counted, they'll still be a ghost when asked to move in.
So yep, such drastic change was made without consulting the previous DLC...
