Choosing grout for two kinds of tile

[Bienes Raices]

As you can see from the photo there is already tile in the kitchen, but the living room (as well as hall & master) need something. I think I've decided to put tile instead of laminate in the bare areas.

Since I won't be able to match the color of the old tile, I was told to put a marble transition piece where the kitchen meets the living room, to make the change in tile less jarring. But I was wondering if I also need to try to match the grout color for the new tile to the grout of the existing tiles?

Also, the tile in the kitchen is 14 inch, but the tile in the bathrooms and bdrs is 18 inch. I was thinking of just using 18 inch for all the areas that need new tile. Will it look strange to have 18 inch tile in the living room running up against the 14 inch kitchen tile? Do I need to lay the tile diagonally in the living room, etc. to differentiate the look? This will become a rental property. Thanks.