Just a suggestion from a fellow game artist.
Remove the mushroom sides from the new stair assets and name the "Acorn Stairs". Give them the same property as the standard grass stairs, in that you can add the various material side pieces to them making them much more versatile of a building piece.
Now instead of them just being acorn steps with mushroom sides, they can also be acorn steps with stem sides, grass sides and concrete sides as well.
Hope this helps...Cheers and Happy Holidays!
-ps thanks for a great game!