For all those other blocks that are not clear:
If the physics of a block is different from the other road types, then this block counts as an own surface.
e.g. glass blocks have physics that are different from all other blocks.
For the scupltures, well they are very unique, same as the black/white 8-5-x blocks. I would both count them as a unique surface.
With the white roofs it is more difficult. I would count them as an extra surface, although they are quite similar to the regular arena platform. As long as you don't exploit the flat 1x1 white roof block in combination with arena platforms, it should be fair enough to count white roofs as a unique surface.
example for exploiting: 5sec arena platform-> one flat white roof block ->5sec arena platform-> one flat white roof block...
The underground roads are a unique set of road too, so they are their own surface as well. I mean, what other stadium blocks are there that are underground?
EDIT: Regarding the minimal track length. Well I don't want to set a number for that, because you should be able to see on your own which length counts as a decent track or what not. With a 15sec map you're definitely not getting far in this theme