Is there a way to use tilesets from all types of maps? Because I want to add stuff from Grassland to Swamp, and I'll want other stuff for other maps.

You can combine several tilesets with an image editor. Things like that for instance: All-round exterior tileset.

So how do I take that tileset and add it to my folder?

First, you'll have to download it. Then use the import button on the editor.