From what my friends have said, year 11 biology is quite content heavy, but as long as you regularly revise and don't fall behind, it shouldn't be too hard. Also as you said you enjoy and are good at science, I think you should be fine. From what I know I'm quite sure for unimelb and monash their science and biomed courses all require either methods and spesh, or methods and another science, tho for biomed you need chem but not bio, but it is recommended. I'm not too sure about other unis tho. About legal studies, you don't need it to do law, but it wouldn't hurt. From what my friends said legal isn't too hard, but there is quite a bit of reading and writing involved, so it also depends on what you prefer.