I have used Forest Pro with Arnold and ART. It works!
There are 2 major setbacks:
1) You have to spend ages of your time to convert all tree-materials to Autodesk Physical materials. Also some things in Forest Pro need Forest material to work, but it is a minor problem
2) There is no proxy support, and it can be a problem with large 3D forest models. ART falls first, Arnold can stand much longer before crashing. Billboard trees work quite nicely in both renderers!
Give it a try! In some point you have to convert all your mr-materials to Physical Materials anyway.
Mr renders Physical Material beautifully, so there is nothing to lose if you choose to use same library in max2017 mentalray. Just remember to make material conversions and library savings in max2017, max2018 libraries are not readable for max2017.