General Studies(GS) - Combined Plan questions

I have been admitted to Columbia GS for Spring 2023. I am planning to apply to Combined Plan in SEAS. But would it be okay for me to apply to CS major even though I will major in CS at GS? I want to have access to SEAS-only research opportunity and also I wanna try different CS track. But would I be in a disadvantage if I apply to same major as I am majoring in GS? I am an international student and I won’t apply for financial aid.
Also, can I apply to combined plan during my junior year if I satisfy the course prerequisites? That is, we are not expected to graduate in the year we get the result from Combined Plan, right?