I would always prefer to use porcelain for floors purely because it is alot tougher as wedge69 says ceramic does crack if anything is dropped hard on them but saying that if they are laid with a good bed of adhesive then cant see any problems.
Adhesive between the tiles is a deffo no no the easiest way to keep it from doing this is to lay your first level line of tiles then use your trowel or scraper and run it along the top of the laid tiles,this stops any adhesive moving into the joint when you put the next line on.
If you get lots...