Like for a maul type game?
Well I'm not sure how to make the grass act as a boundary to just one unit, but you could make the grasses into a region(s) and whenever the unit/type of unit walks in it you move it out? I mean, that won't make a unit go around the grass as if it was an actual boundary.
In the Doodad Palette you can add Pathing Blocker (Air) and Pathing Blocker (Ground) around the grass and cobble. You could put pathing blocker air around all the cobble path and change the units on the cobble path to the air type to restrict their movement to just that path, and same for the grass with ground. Dunno if this is what you want but it's another option for you.