
Hi Donna
No one will be able to tell you for sure if your rottie will get on with new pup. If he is just a bit grumpy and not actually aggressive I would say he will be fine.
It would be a safer option to go for a bitch, make sure he has his space away from the pup and don't force puppy on him, let him come round in his own time, he will but it sometimes takes a few weeks. Lastly don't leave them alone in the same room unless the puppy is crated.
Your older dog being six years old is a good time for a new pup, he is still young enough not to get stressed by a youngster, but there is a large enough age gap where you have one that is hopefully sensible with a degree of training behind him.
Good Luck
Sandra