I've never placed a mat on the pre-existing ground plane, but rather I bring in a single sided plane from the Prop/Prop Types and mat that.
I do not knowing Bryce that well, but I'd say Yes. Bring in an object that has a mat assigned, and LOOK at how it is being done via the Render/Material. There is a Texture Map, a Reflection Map, a Bump Map, and a Transparency Map. Not all objects need all these functions, but it would be best if you looked at various types that do use each of the variable to get a better understanding.
Yes. From your previous PZ3, save the objects you would like to bring into your New PZ3. See the '+' sign which will add the object to a folder of you must 1st create. I use !AAA so it stacks on top of all others, and I create a separate folder in each of the Runtime/libraries Folders.
ie. a Character, and all parented items can be added to the Runtime/libraries/Character/!AAA
ie. a Prop, and all parented items can be added to the Runtime/libraries/Props/!AAA
Be sure to select the individual items you want inside the add dialog box.
You can also save Character Poses, but not Mat Poses. I do that via copying an existing Mat Pose (or extracting the Material section out of a .cr2/.pz2 file) and replacing names via WordPad.

Forum: Poser - OFFICIAL

Subject: Creating scene in Poser