I've done some Inconel 625 cladding with pulse MIG and found that a tri-mix of He-Ar-CO2 worked well, but provided too much penetration for cladding, but for a structural application, it might work. I think something like 50-75% He, 1-2% CO2, and balance Ar would get you a bead that will wet and penetrate. If you've got a gas mixer, grab a bottle of 98-2 and a 100% He bottle and see what shakes out. You don't need much CO2, it's just there to stabilize the arc, you may find that you don't need it at all. The thing about Cu-Ni is that it's sluggish and with MIG it'll won't want to wet. Let me know what you end up using, I might have a project coming up that I could use pulse MIG on Cu-Ni.