We have a gorgeous 2 year old collie x who came to us as a rescue pup. He loves to play with other dogs and so we'd like to get a second dog this summer. While we would be happy, in theory, to have a rescue dog which isn't a puppy, our current dog is very submissive with other dogs and it would be nice if he could be top dog at home. For this reason we are thinking it may be better to find a puppy so there is more chance of this happening. Any thoughts / suggestions / advice?
Tbh, I'd say there is more chance of that happening if you get an older dog, whose personality is already formed.
I think though either way, when you get a second dog you need to let them work out who's who and where they stand. Girls will often be the 'boss' over boys for example.
I have known friends get a new pup and the established dog be fine with it until it reached maturity and then the younger dog decides to challenge the status quo. Otoh , when we got or second (older) rescue, we had none of those issues and the two dogs have got along well from the start (with the occassional exception, but never a full on fight).