For p7 it was possible even before the rod upgrade as Washly and Glue proved in their 4k% duo 5-man scaled kill. I don't think it will make any differnce for p1-6 other than saving you from a mistake. However it does make p7 much safer on the 2nd and 3rd cycle as you can dps for longer before dying/signing/immorting
You can't use protection prayers while teamwork protection is active. Soul link isn't really worth it, as we both are going to take and heal the same damage in the end anyway. We do use intercept a lot instead to help eachother with tanking some parts of the kill.