I intend to double major in CS/Math and go to grad school for TCS. I have some - unorthodox - research ideas I would like to pursue for grad school. At the very least, they are interdisciplinary (think akin to humanities/STEM fields). Would this be a red flag for a AdCom for CS departments - as long as I can articulate why this research need to be done and why I should be admitted for it ? I know some other academic departments loath interdisciplinary more then others, so I want to know if CS/TCS is one of those.
If there's any 'poison pill' topics, I want to know so I can decide whether to pursue grad school.
I can mostly on speak to the US (and maybe a little Canada). In most CS departments here, you are admitted to work in a particular group (e.g. "the theory group") or with a particular professor (or maybe small group, e.g. if 2-3 professors work very closely together), not just "to the department" (which is, in contrast, how it often is in math departments). So for that type of dept you have to find a professor or group who would be interested to advise you and help advance the research you want to do.
Many TCS researchers and pure mathematicians unfortunately look down their noses at "complex systems" research; that is one of the few areas I can think of where this stigma is common. (Not me, and not my dept.) Some departments are more open to / encouraging of interdisciplinary research than others. Many say they want interdisciplinary research but their view of interdisciplinarity is very narrow. Look at what the professors and students there work on to get a sense. (And you'll want to do that anyway because of what I wrote in my first paragraph.)