We just got Rexy a month ago (he's 13 weeks) and Rowdy is 14 months old. We do have a senior schnauzer who was here when Rowdy arrived.
We took Rowdy with us to pick up Rexy so they met on neutral territory. We did exactly as someone else mentioned, greet Rowdy first, give him the attention he was used to having, and definitely made sure he felt like number 1! Little Rex didn't seem to mind at all - he was kinda oblivious for the first week or so. It was hard, but we didn't pick up Rexy everytime we saw him - although I WANTED to. I just made sure that Rowdy felt secure.
Rowdy was never mean to Rex, he didn't growl at him or anything and they were playing in a couple of days - but in general, Rwody loves EVERYONE and EVERYTHING.
Also, when Rex would be in my lap, Rowdy would come and get between Rexy and my face (yes - he would sit on my boobs!)
I just let him and I didn't make him get down. Rowdy still does that sometimes, but not everytime I hold Rexy.
Everything has worked out well. They play - but do get a bit rough at times. Like another poster said, I sometimes have to rescue Rowdy because I dont' think Rexy realizes how rough he can be. We still have an occasional squabble over a toy, but basically they are best of buds!