Obviously there is no guarantee, but also keep in mind that there are probably other transfers. I knew 3 transfer girls to very varying degrees the past 2 years and they adapted pretty well, I think,. Of 3 boys I sort of know, 2 I think adapted pretty well, the third I don't know since we lost touch since he was 2 years older than me and we weren't in any classes or clubs together. I know of 2 girls who were in a forced triple my first year, one considered transferring out of Brandeis, one of them a good friend of mine, because of some of the latter social concerns you mention, but in the end she chose not to and I think was happy last year (she also had a single as compared to a triple) and is going abroad next semester. The other is transferring to Brown this coming semester. There are also definitely many people who are not the hard partying type. What kinds of things do you like doing in your free time for extra curriculars? What type of clubs do you think might be interested in joining? Do you think you will have a job? Those are good ways to meet people. I'd say most Brandeis students do work hard, take their studies seriously, but balance work with other activities usually of the extracurricular or hanging out kind, not necessarily the partying/drinking kind.
ETA: You can see a list of clubs here, if you haven't already:
Club Center