I'm afraid there is no only one best way, but most usually having items with similar size and using corresponding Density
(they should overlap slightly) is the easiest way to prevent troubles like the described one (the Area
isn't covered completely).
That doesn't mean, there is no way to fix that behaviour by raising the Density
(setting low numbers) and enabling the Collisions
feature (feel free to check use of this work-flow for a slightly different purposes in following "crystals population
" post), but that's not very efficient way.
The recommendable work-flow of building "large" presets would be to attach those "detail" (smaller) objects into few clumps with similar diameter like in case of Unkempt Lawns
presets. I would suggest you to give a try the Distribution Map
feature, which you might find handy for this kind of presets.
Also it's possible to "divide" it into several Forest objects (as it's actually done in case of VP Real Grass presets) and tweak adjustment for those "layers" (Forest objects) separately.
"What ist the highest quality grass katalog out there? Evermotion, Maxtree, Real Grass?"
The quality of all mentioned vegetation models is very high and it's hard to say which models suits best to your scenes. As there are free samples available, probably the best way is to do some tests with them and than decide.