In Nintendogs+Cats, it seems like cats/kittens are only an add-on to the main game of dogs/puppies. If I only want to play with cats, how much of the game will I miss out on? Can I take cats on walks?
You must first adopt a puppy before you can adopt any kittens. Kittens cannot walk or participate in competitions.
If let loose in your room they may find surprising treats, but without money coming in from walks and competitions, you might find yourself low on money. This would limit the amount of toys and other purchases you could make in game.