It can either look really dramatic or too dark, depending on the size of the room as well as your other components. I've attached a couple of images of our work using dark tiles, the smaller bathroom we have surrounded the bath with dark tiles but then balance it with an extra large mirror to bounce the light around the room. The larger bathroom we have taken the floor colour way onto the wall but used a different texture on the walls for drama. Hopefully this gives you more of an idea but you shouldn't to nervous of the room being dark, it's where you can create a contrast as well as textures. Look forward to seeing what you go with in the end.

I agree, I think it will look lovely.

With most old houses walls and floors are not straight and perfect, however wall tiles can go some way concealing this. I don't know if a reputable tradesperson will fix glass panels to old tiles and be happy to guarantee the job. Seems like an ideal opportunity for mould growth between the old tiles and the new glass if any shower water gets an opportunity to seep around the edges or joints of the glass. I think you may be surprised just how much this type of glass costs to install, and you would probably be better off doing the job properly with new tiles and a proper waterproof membrane as required by the building regulations (which your old shower recess doesn't have). In the future your potential new buyers will probably appreciate a freshly renovated bathroom that conforms with the current building regulations (with standard tiling) rather than a glass panel over mould in a bathromm that doesn't comply with the regulations. Best of luck, Dr Retro of Dr Retro House Calls

If the hexagon tiles are to be the feature then the wall tiles should not draw attention to themselves. I would go with stacked horizontal as this will make the small room seem wider and it will be a simple grout pattern that won't try and compete with the hexagon mosaic. For small bathrooms understated and simple always looks cleaner. Best of luck, Dr Retro of Dr Retro House Calls