We have DJ (Bruce's Cocker) and Taz on the shelf along with our pictures and things. We kept the cards we got from the vet office and DJ's collars and a toy.
My sister has 4 boxes up in her old room at my parents house of all her dogs. They were at my parents when they went to the bridge so they are all together in her room.
Personally, I wouldn't mix them together. But that's just me. I don't see anything wrong with doing it, I just wouldn't want to. I like having their own separate places.
When my brothers dog died this past May they had him cremated only because the ground was too hard to dig a grave for him. So when the ground softens up they will bury his ashes along side his buddy, Digger, who passed several years ago. They live in the country with acerage, that's why this is possible.