Well, like many things it depends. During your big rotations there isn't a lot of extra time to spend on research except perhaps a bit of writing here and there. However we had 8 weeks of elective 3rd year, that time could be used for research. As far as putting off core rotations, some schools will let you do this... but be careful. Don't put off medicine or surgery, but 4 weeks of neurology etc (we have a required 3rd year rotation) would be ok to delay if you felt like you really wanted the research time.