Introduce on neutral ground. Leash both dogs and take them for a low stress walk. Keep them moving and let them get used to each other on neutral ground like the park. Repeat for several occasions. Give them casual introductions.
Signals to watch out for: body stiffness, lips curling up, low deep snarl or growl, ears going back, dog getting lower on the ground like stalking, tail stiff and held straight out, hackles and hair standing, eyes glassing over (they will appear to have a sheen), dog looking sideways and holding the head stiff...those are all signs that indicate trouble in the right context.
Based on the initial introductions, continue to take it further. Introduce off leash on neutral ground as well. Always keep them contained and be able to manage the situation. Don't give toys or food when two strange dogs are around. Watch out for guarding or defensive behaviors.
My biggest question is why do you want to introduce them? The dobie is 9 and you do not know his background or what he has experienced. Now is not a good time for a newly adopted dog to be exposed to multiple new scenarios. Also - I do not think GSDs need to be friends with dogs outside their home. I am not a big fan of doggie play dates, dog parks or these type of situations. More trouble than they are worth.