Seniors are the best. Larger dogs are also pretty mellow but usually when they are older- around 7 or so

I would say go with an older dog. My parents have a 10-year-old beagle who does nothing but sit in his chair all day. He jumps off when anyone comes home, gets his belly rubs and then gets back in the chair. Yet, beagles are suppsoedly "high energy" dogs.

Daschunds are so lazy. That's one reason the stereotypical "little old lady" is a daschund-owner instead of a Jack Russel-owner.
They are good snugglers, too.

English bulldogs are generally really mellow, lazy dogs. The fact that they are the most popular team mascot makes me laugh. The downside is that they are prone to a lot of health problems, which can be very expensive.

It seems that the larger the dog, the more mellow. Counter-intuitive, I know.

Would you be open to adopting an older dog?

Also, the more exercise a dog receives, the more mellow the rest of the time. We like to say that a tired dog is a good dog.

Is there such thing as a low energy dog?

We have very laid back (lazy) persian cats that just sleep and eat most of the time.

We're thinking about a dog too, but not sure if it'll be too much for the cats.

Any recommendations? Preferrably small, but not toy dog. Our cats are about 9lbs each.