1) Yes, FP6 can handle older scenes without problems. As usual, backward compatibility is not guaranteed (open FP6 scenes with FP5).
2) To difference of V-Ray, it's a bit more difficult to keep FP5 and FP6 installed at same time, because we use an unique installer, and both versions share a common folder and several registry keys.
But it can be done in the following way:
- Install FP5 as usual.
- Be sure Max is closed.
- For each Max version you want to use FP5, rename the Max root folder to any other name (as C:\3dsmax2017 -> C:\3dsmax2017_a)
- Install FP6.
- You will find the setup program has created a new c:\3dsmax2017 folder, with the FP files within it. Just delete it.
- Rename "C:\3dsmax2017_a" to its original name.
This is required because the setup programs uninstalls existing FP versions, before installing a new one. Renaming the Max root folder, we avoid the FP files for that Max version are deleted.
With this process, FP5 will use the common files of FP6 (stored at "C:\Program Files\Itoo Software\Forest Pack Pro"). But that should not be a problem, except for the new libraries.
Also it may be a good idea to make a copy of the FP5 common files, before installing FP6 (at "C:\Program Files (x86)\Itoo Software\Forest Pack Pro").
We have replaced the old "HQ Plants Free" by our own library. So you may need these models or maps for older scenes.
3) New libraries will be included only with FP6. But they may work or not with FP5, depending of the features we used on them (really we have not tested that).