There are mods that allow you to control mob/ore spawning and mods that allow you to control cave generation and mods like minetweaker.
Is there any mods that allow someone to control small plant generation?
That allow you to set which dimensions and biomes specific plants generate in?
Ive tried searching but ive come up empty...
I think some of the terrain generation mods might be configurable for that. For example, I found that TerrainControl lists as a feature:
Full control of ores, plants, dungeons, ponds and many more