Righty - ho .. Firstly, pedigree is known parentage, and parents are of the same breed - eg labrador. Cross-breed, is a mix of 2 breeds, parentage known - eg labradoodle (lab x poodle). Heinz 57 is an unknown mix the dogs that have a bit of everything.
It doesn't matter if you have a garden or not. Really a garden is for wee's and poos and a bit of play time. I'm afraid a garden doesn't make up for daily walks, they are needed no matter the size of dog. Walks are more than just exercise .. you have socialisation, mental stimualtion and a change of scenery for pooch.
The thing about big dogs = more exercise, little dogs don't is a myth
If I had a penny for every person who has said to me "Oooo I'd love a collie, but couldn't deal with the amount of exercise, they need hours and hours of exercise everyday" I'd be very rich by now
You'd still have to take a chihuahua on a walk everyday come rain or shine I'm afraid. A tired dog is a happy dog. Just playing with a dog doesn't usually tire them out for long .. getting them to use their brain does!! Some rescues do rehome to people in flats or with no gardens. A lot of times thedogs are looked after better than people with gardens, they get more walks for toileting. A lot of people use their garden as a substitute walk - which never works.
Based on your situation I'd recommend a dog age 2+ that way you've got past the houstraining, which is sooooooooooooo much easier in an enclosed garden and you've got a dog past it's adolescence which is a nightmate. At the age of 2 dogs start to settle down. Personally my favourite age for a dog is 5
Bostons will need a long walk. I know they are small but they are active.
Out of the 3 breeds you mention Pug would be the best. They need the least exercise and you have to be careful you don't overdo it.
Pomeranian would be good provided it could run somewhere such as in a park.
If you are up for a challenge you could try the Papillon.. should be good with cats. A bit of a yappy breed, if you live in a quite area with quiet neighbours there shouldn't be a problem.
Miniature Poodle - are good with cats, active indoors, intelligent but are barkers.
A Cavalier King Charles Spaniel could be perfect ... great with people, kids, dogs, cats. Affectionate, like cuddles, they need a walk but will settle indoors easily. Not real barkers .. they will bark but less than other breeds. And they are small.
Bichon - Likes people, other animals, they are small, moderately active and love travelling
I think all those breeds could work in your situation .. all are small. If I was to narrow it down even more in order of best to worst I'd say ...
1) Bichon
2) Cavalier King Charles
3) Pug
4) Papillon
You should be able to find all breeds in rescue ... unfortunately if you wanted a Boston you'd be looking at buying a puppy from a breeder as they are rare in rescue and there's usually a long waiting list.