Subway tiling is apropriate if the tile has been sanitized and the property is yours id you own your home. I would use a concentrated solution of bleach to soak the tiles in for round 24 hours before laying them.
It depends on the thickness of the tile. If the tile is to thick it will cause problems in the lay out of the tile. Check different tile stores and see what you can find. Speak with the tile persons and see if the quality you want to pick will work on the entire shower.
No, it will make the shower slippery, very slippery, any kind of semi-gloss tile is not appropriate for using on flooring that is consistently wet. To prevent slips and falls, only use it on the walls.