Your Project is realy interesting.
to place my trees I´m usind the Hair Partice in Blender and export the Models as one Model, the limit of vertces is <64k, more and my system crashed.
Hair Prticle are very simple for beginners, du you have experience with blender?
The Basik Steps are these:
klick on the Gras models right and press Strg+G, to group your Models Pool.
Chose your Terain Model and (1&2in picture) add a Particle system.

in Type: you chose Hair.
in Renderer you chose Group chose in the empty mini window your Group with grasses.
Ready.

You can make/chose many different settings:
Random Object from the Pool group
Random Size
parant by normal Angle or not

at last you have to make hair to vertex convert, sorry I don´t konow how this funktion is callig but you find it in the left down popupmenue.