I guess this question isn't specific to the fillable pillars but pillars in general. These things don't come with instructions other than how to fill them. I realize that on the bottom cake I need to use a plate separator? Now, the cake will be covered in butter cream and won't have a ton of decorations in the area where the plate will be. Can I ice right over it? Or should I cover it in fondant (some other aspects will be fondant so it shouldn't stick out)? How do y'all hide the plate? I don't want any of it showing..... Thanks!

I think it will be fine uncovered. I would make a small border around it or it could stay just as it is. Sometimes there aren't enough decorations to go around but, if you can put just a couple if you think it would make a little more elegant. Either way it will look beautiful.
evelyn