you are correct that the ko servo and receiver can tolerate the 2s lipo voltage but NOT the tekin rs.
you need to add a booster to the 1s battery, or use the 2s life battery, or do the BEC wiring trick on the picture above.
I've actually been running the same RSPro for three years with a 7.4 lipo plugged directly into my rx. MAy not be recommended but I haven't had problem one, and I race twice a week.