Sorry, thought you were still talking about wood
Should be fine....although I would take a trip to the garden centre and pay a lot less for a lot more...granite/slate is fine...
I see now what you mean about the mirror, as you wont be able to view the tank from that side why not get some background sheeting and wrap it from that edge and then around the back...nice black background would be nice too...that wallpaper is a tad bright!
I'd say no to the black widows and instead get something a little more inclined to stay at the top of the tank...don't dismiss Danios...they're fantastic to watch when they get into a breeding/mating frenzy...and they can handle 26/27C just fine...and they're cheap! If you can get some Celestial Pearl Danios they look really colourful too! Or just drop the black widows and don't replace, then get more rummy noses
I'm now thinking about getting some dither fish for my big tank, I'm thinking Silver Hatchets at the mo because they'll stay right up the top and are SA fish too to match everything else. A bit strange looking though