I actually went to Brown as an undergrad, and while I enjoy your enthusiasm, I have to agree with Rafa. The PLME students may try to avoid each other, but they pretty much can't, due to special advising groups, PLME-related conferences and activities, special meetings and opportunities, etc. They definitely have their friends outside of PLME, but they also are a close unit. There are closer to 60 students each year moving into the medical school, seeing as Brown has only recently tried upping admission of "outsiders," so the majority of your classmates will already have known each other and done things together. Not to say that they won't welcome you, though. A medical school class as small as the ones at Brown can't help but become a family-type unit, but keep in mind that the PLMEs do know each other, and know each other quite well.
The situation at Brown is very unique compared to that at other A.B/Sc.B/MD programs throughout the country, as one of the few things that students had to do to get into the program was write an essay or two in addition to their application. And they don't really have to do anything to stay in the program (such as maintain a certain GPA or take the MCAT). All the have to do is fulfill the requirements of their respective degrees and graduate the college. They are really looked out for by the Medical school in the way of academics and support systems, which is a lot more than some undergrads could say. This will bode well for you. As a result of the very relaxed nature of the PLME program, many PLME students have been able to relax throughout their four years and really concentrate on enjoying their education rather than "the next step," which is what many of us have (and will) go through. Some have taken advantage of that, and then there are the others, who are just anal retentive and dead set on beating out competition they don't have...but where wouldn't you find "the others?"
That said, you're going to have a fabulous time at Brown. Why? Because you really want to be there. Four years is enough for anyone at Brown, and even some of the PLME kids will tell you the same. I know quite a few who have opted out of the program and chose to apply elsewhere. Others know the stress and choose to stay, though Providence isn't their love. It's just a smart decision if you don't want to be a true premed. I'm sure you'll make the most of your time there and make it fun for yourself if no one else will.
Oh, and that new research building is the Life Sciences building (call it LiSci to blend in) for all of the BioMed department, not just the medical school. The one off campus (near the hospital) is also for Life Sciences, but not brand new. But yeah, change is a comin' to Brown. I can't really speak on Tufts, but I love Boston.