I wouldn't worry too much about it right now. First of all, it's well known that the vast majority of people you meet and speak to during freshers are folk you will never see again once that week is over. In my experience most people's friend groups end up comprising course mates and/or flatmates, so make sure you put in the effort to get to know people on your course and be prepared to stay out later for social events with them to solidify the friendships if possible. Concerning first year hall mates it's a bit of a mixed bag, I know sooooo many people who after first year never spoke to their hall mates again and ended up getting second year flats with the besties they made on their course.
As for your friends going off to uni with people they know, this isn't necessarily a good thing. They may stick to each other like glue and possibly not put themselves out there and meet as many people doing that. Also, if they're on different courses chances are they won't actually see much of each other. Most undergrads start uni ''alone'', just remember that all the other first years will be in the same position!