Those are all really interesting topics, but I doubt most of them would have any solid results since reptiles all have different personalities. I feel the results would be specific to the individual reptiles I studied. I could get solid results if I had maybe 10 or so, but that isn't an option obviously. This isn't a formal research project where I need to have as solid a topic as you suggested. It's a little hard to explain. For a capstone you need to choose a topic specific to your interests. Some people host something as simple as sports tournaments and make that their capstone. So the topic I choose is more than acceptable. I already spoke to my guidance counselor to ensure I could do it, and she said yes. So you could ask, "Why don't I just make a vivarium then?". I very well could, but I feel I would get better reception with this topic. I wanted to challenge myself a little more, and on top of that I'm not fully confident a vivarium would be anywhere near as good as my current idea. I have a lot of confidence in this cohabbing idea. I am yet to find any people that have a failed cohabbing enclosure for pink tongue skinks. Even when people give them less than the suggested space they seem to live peacefully. So your suggestions are interesting, but they just aren't appealing for this project.
