For the plane’s dynamics tag, change the collision “Shape” to Static Mesh (or Automatic (MoDynamics) or Moving Mesh). Right click on “Shape” and select “Show Help” for more info.
That is normal as of R20. To scale a texture, either use the scale tool in the viewport in Texture mode, or adjust the scale settings in the texture tag. Or is there something else you are trying to do?
To minimize the look of triangles flying apart subdivide the mesh a few times — Mesh > Commands > Subdivide (or U > S). In my example below I subdivided twice and applied the Explosion object with a speed of 10 and animated the Strength from 0 to 100% over two seconds.
The Explosion FX is not breaking the mesh into triangles — the mesh of the figure is already triangles. Explosion FX simply explodes them apart. Try the Explosion object with the speed set to zero, and animate the strength from 0 to 100%, and see if that’s what your after.
The Explosion FX doesn’t change the mesh but only works with the mesh that it is given. With the Explosion FX disabled (or before applying it) select Display > Gourad Shading (Lines), or keyboard shortcut N > B, to see the mesh. You will then see that the figure’s mesh is made up of the same triangles that are used by the Explosion FX.