Well I'm from Hawaii (as warm, humid and tropical as it gets) and spent a semester in Sevilla, including January, and it was not bad at all....although I think that the weather was exceptionally warm...but I didn't really have a problem with the cold.
As for meeting international and Spanish students, Sevilla is as good a place as anywhere. TONS of students go there to study (mostly americans though, don't know what kind of international students you are interested in meeting). It is also easy to meet Spanish students. So don't worry about it, you will have a great time!