If you search around this forum a bit, you'll be able to find more detailed/diverse answers to some of your questions. But for my part and observations as a pre-frosh and brother of a rising senior at Pomona:
1. Social life is perhaps lower-key than a larger school. By his junior year, my brother knows everyone in his class by name. He says that most people form into social groups throughout the years, though they're by no means hierarchical/clique-ishly exclusive. They're primarily coalesced around shared interest or another common factor (economics majors, ultimate frisbee players, etc). Of course there are overlaps between these groups, but you get the idea: the school is small and though you will know everyone, chances are you will have a close group of friends for most of your time there.
As for nightlife/parties, there are a number of school-sponsored parties at each of the Claremont Colleges (Mudd parties being the most popular). Otherwise partying is mostly done in small group settings. Claremont is a pretty quiet town, and most things aren't open very late, even on weekends.
2. There is virtually no Greek life at Pomona. Don't attend Pomona if joining a sorority/fraternity is an essential part of your college experience.
3. Yes, there is quite a bit (though arguably it is done by a vocal minority if I understand correctly). There was a bit of controversy last year between (some of) the students and the administration over the hiring/firing of undocumented workers. If you google around a bit, you can learn more.
4. All majors are pretty good, though some, like Politics, are easier (fewer required classes, no thesis, etc). I would say biology, economics, and math are the top majors at Pomona. Of course, all majors are made stronger by the inclusion of the other 5Cs (for example, the Computer Science department at Pomona by itself is relatively small, but is partnered with Harvey Mudd's).
5. Yes, but I would not consider it much of an advantage. There will be plenty of highly qualified applicants from your state/area applying to Pomona.
6. They're incredibly different schools. I'll list what I can.
- Though adjacent, their campuses are very different.
- At CMC, upper/lowerclassmen are not in separate housing, whereas at Pomona there are freshman-only dorms (with sponsor groups), and the upper/lowerclassmen typically live at north or south campus, respectively (north/south campuses are separated by the quad and some other facilities). So social dynamics are affected slightly by this.
- CMC is a little more 'involved' in the 5Cs, whereas Pomona is a little more removed/separate/independent.
- Pomona students tend to be more intellectual (NOT a comment on the intelligences of the student bodies) and interested in academia, whereas CMC students tend to be more practically focused.
CMC and Pomona are very different schools - it is hardly useful to compare them in either their similarities or differences. If you are considering both, visiting both schools will be the best way to decide which is a better fit for you.
I hope this helps!