Galvanized works. But for homes, you would ideally have less steel in your well system. Galvanized is cheap, and your thought is that you already have the drive pipe as galvanized, so a little more won't be a big deal.
You need one what? A 1-1/4 inch coupler below the packer? Are you saying that the packer that you plan to use has a 1-1/4 male thread on the bottom that you must adapt to? Many well devices have a 1-1/4 male and 1 inch female thread on the same device. Check yours to see if there is an inside thread too.
If that is the case, you could forego the tailpipe, and instead put a 1-1/4 foot valve right on the ejector.
Many "deep well" packer ejectors come with a foot valve already installed.
Note that I have no relevant experience, so use my post to identify things that you will search further.
EDIT: I am now thinking you are referring to the top of the packer, but still near the bottom of the well. That changes things. "Never mind"