When playing with room mode Tiles (Any/green or acid) can only be placed in spots what don't have any tiles. You can't seem to replace colored tiles or the concrete ones between. However you can use paint the tiles anywhere within the yellow boarder lines as long as there isn't anything there. I'm unsure if this is intended because it seems to make the use of acid tiles troublesome and the "any" tiles don't really work with the rooms. I assumed they should replace the colored tiles in the rooms but I could be wrong.

As for the the cave planet boss that paths along a line I noticed it pathing over a wall and concrete structure that had a inserter, pipe, and chest weirdly in the center of each room. The issue was that because the wall/concrete room didn't have rocks around it the boss the walked over them into the void. Might be just a chance event that isn't worth the trouble of doing something about, but I thought I should mention it anyway.