I will be beginning a science project about sustaining life on space stations. I already know that pure oxygen is harmful for humans but would $O_2$ + $CO_2$ be enough for humans to breathe in or do humans need other gases as well?
Why not look at the existing space station as an example. The International Space Station (ISS), a starting article is "Breathing easy on the space station" (NASA), while this article does not discuss the amounts of various chemicals, it looks at the process about how the chemical balance is maintained - an important point related to your question is that carbon dioxide is filtered away.
The second article "Breathing on the Space Station" (e-missions), discusses the actual percentages of each component. It states that the chemical composition of the space station air is ideally similar to Earth's atmosphere. With 78% nitrogen, 21% oxygen and 1% water vapour on the space station (Argon is in the Earth's atmosphere, but appears to be unnecessary on the space station).
I hope this helps.