Well, I really don't know what to suggest.

Puppies need alot of care when they come home. They need potty training, crate training, regular training. They need alot of attention and as you mentioned they need to be watched constantly with a larger dog around. The thing is are you willing to take the time out of your day for this?Time off of work or out of your daily schedule to train this pup properly. Of course, there is a plus side. Unlike older rescues you can train them the way you want from an early age, which is good.

Older dogs are also a good choice. They can be left alone more and are most likely potty trained(especially if you adopt from a rescue.)Some are already trained and they will already be large as your other dog is. The down side is, what if the had a bad past? Sadly abuse is always a possibility. Rescues sometimes don't check how the dog is with other animals. So what if you bring him home and your other dog gets into a fight with him?

Like Bckrazy said, it's really your decision. But make sure you get from a recuse that has checked thier dogs with other animals and children.

Good luck in your search!