Good question.  There are no studies that I have seen on this.  But from talking to some of my collegues and my experience, here's what I'd recommend.

There doesn't seem to be any problem at this point with using different fillers in different locations on the face.  For example, using Sculptra or Radiesse to provide some volume in the cheeks but using Juvederm in the lips or around the mouth.  

The tougher question is whether you can use different fillers (sometimes called layering) safely in the same location (your question).   I would strongly recommend against layering a permanent filler like silicone or Artecoll/Artefill with anything else.  What if the other filler "activated" in some way the permanent filler and caused inflamed permanent bumps?  Not a good scenario.

I would be very cautious about layering other substances.  Sculptra lasts about 2 years with 1 maintenance treatment once a year usually.  With Sculptra who wants to go from full to zero?  It's better to "top off the tank" once a year to maintain a fairly stable volume.   If you then inject Radiesse or Juvederm on top of or into it, could you cause lumps or "activate" the Scuptra in some adverse way? I don't think we really know.  It makes sense to me that if, for example, in your nasolabial fold, Sculptra is injected deeply and then Juvederm layered in a different plane in the skin altogher, they wouldn't theorectically interact. It takes an expert injector to do this accurately.  I also think Restylane/Juvederm is safer because, if there is a problem, it can be dissolved with the enzyme hyaluronidase.  

Why not just stay with a one treatment once a year maintenance schedule with the Sculptra, if that's what you choose to start with?  Hope this helps.  

 Dr. I