If you want a polygon's texture to be unstretched (normal), select the polygon and hit [CTRL] + F (Fix texture; also accessible in the Edit dropdown menu). If you want to manually stretch a texture yourself, select the desired vertices and use the Texture Tool.
What do you mean by "solid objects?" Colliders stop projectiles and explosives. There are different kinds of polygonal types which you can find in the Properties window. If you're trying to achieve climbable scenery or whatnot, set the scenery to the Front layer and create polygons behind it.