I know that Carrara 8.5 Beta can use the new Daz Studio DSON stuff, and 8.1 can use Poser content, but can either version use the old Daz Studio Legacy content, from before DSON was invented?
Yes. You mean Michael 4 and Victoria 4 and such? Buildings, props, dragons, horses, elephants, cars, spaceships, clothing, hair… all of it.
The stuff that doesn’t work in versions previous to 8.5 beta, would be anything Genesis related.
Also, Daz Studio-specific things, like Movie Maker, or Optitex dynamic clothes, Daz Studio shaders, lights and cameras… that stuff doesn’t work in Carrara. At least to my knowledge and understanding.