Thank you all for your replies =)
It's just a couple of pixels that are missing and I'm happy with my results, using these new tiles.
Here is a new picture that hopefully explains it more precise... the wall on the left side is built by using new tiles and the wall on the right is built by overlapping tiles.
The first thing that happens when you overlap tiles is that there are three pinnacles instead of two (blue circle). Second is that there are some pixels cut off the edge so that the corners are not so smooth (red circle). Third, there is missing a tile without pinnacles (green rectangle). That might be not a problem for anybody at all, but sometimes I'm a little perfectionist... sorry for that

Btw: Another thing I wonder about is the difference in the color brightness. Is there a reason why the colors of the solarus tileset are so much brighter than the original tileset colors (compare with the last picture I uploaded)? Concerning Nintendo copyrights, maybe?