Those are also type combos we've yet to see in the game.
I'd like to see more exotic combos.
As far as starting Pokemon goes this is true. However Grass/Dark and Water/Fighting have Pokemon with those types.Shiftry and Poliwrath respectively.
Fire/Psychic isn't in the game at all.
You know as far as starting pokemon go Fire/Psychic has an advantage with most of the Fire starters except Typsplosion and Charizard. Empoleon would really be the perfect counter as far as types go.
Grass/Dark has neutral attacks with its typing goes as far as Grass starters go. Grass/Dark has a hard time with the rest of the starters except water types so I think to combat that I think it would be bulky and defensive.
Water/Fighting would be neutral for most of the water types except Empoleon. Grass type starters it would be neutral for all of them except Venasaur would could resist the fighting type attacks with its Poison typing.