The sport shirts, I think are generally supposed to be untucked, but I was wondering if they can be tucked in as well?
Yes. To be honest I do not see a reason why they shouldn't be. But it does depend on the polo (my way of saying sport shirt maybe due to the fact that I have lived in Germany for the past 5 years and there everybody just says - polo) and on the trousers/jeans/etc. you are wearing. So in general (IMHO) it is def. not a faux pas even if some people might think of it that way. If there is a specific "sartorial case" that you are wondering about then feel free to post pictures.
Twelve models in two photos and not a single sport shirt (or shirt of any kind)...
Mike, any shirt that is physically long enough to tuck in can be made to look good that way. I've come across button-ups that were cropped enough in the body there was no way to keep them tucked in, they'd come all the way out if you so much as bent partway over.