There are less and less reason to develop for the Legendary Edition due to the inherent benefits to the enhanced game engine of SE so mods are less likely to have versions for both today. Additionally, a large limiting factor during SE's launch period was the lack of SKSE (The Skyrim script extender, used for many, if not most, mods) which has since released for SE. A grass mod : The three best imo are Verdosebrom, Verdant and Viscous Foliage. I use the latter for my setup, but pick your poison. The age old debate between ELFX and and RLO. You can combine it with Wonders of Weather and True Storms (with the provided patch) for even better results. If your search is sorted by endorsements (default), the Unofficial Skyrim Special Edition Patch should appear as the first mod in the list for results. Obsidian Weathers: The best weather mod in my opinion. However this may be becoming less and less common considering owners of the Legendary Edition (Which was generally required for most large mods before the release of SE) received free upgrades to the SE when it first released. Navigate to the Skyrim Special Edition section on the website. If there is an easy way to make them compatible, numerous authors choose to release and update versions for both. The article linked by Kyle Rone in his comment on your post does offer some reasons why the versions aren't compatible, but is not technically an answer to your question, since it is the other way round. It would really depend on the mod (some could work 'out of the box' or with light modifications), but generally it is safe to assume SE mods won't work as intended for the Legendary Edition or any other version of original Skyrim.

What mods are in skyrim special edition