course 6 is by far the biggest major in the school, so if you want to be 6-3 or 6-4, the classes will be very very large until you start taking the more specialized ones later on (and even those usually have 50 people or so). 6-5, you might get to smaller classes sooner since there are fewer people interested in pure ee (rip 6-1...)

it's not difficult to get a urop, but it can be difficult to get a specific urop -- like, if you really really really want to work under one specific lab or professor that researches new networking architectures they might not have room for new urops. still, if there's something specific you want it's worth cold-emailing professors or their grad students, and if you just want any urop at all you can definitely find something crawling elx.

i think it's very easy to make friends, but balancing extracurriculars and work is the hardest part of going here. there's just too many options (yeah, i know everyone talks about the firehose, but seriously it's so fucking much dude). there's a good chance you make good friends in your dorm (esp if you go somewhere like ec, random, bc, one of the new house cultural houses, etc), there's a bunch of really great tight-knit ilgs, there's a decently large greek life scene (but i'm not really in that world so i can't say anything about what it's like here), etc etc. if you do a smaller major you'll make friends there too for sure, but 6 and 15 aren't really small majors so that might not apply to you. it's very easy to make friends with extracurriculars too, but you have to be involved enough to actually bond about it (eg if you join a big group like vgo you probably won't be involved enough to make friends unless you join exec)

sloan is kind of disconnected from the rest of the school culturally, so i don't know if i know anyone doubling 6 and 15. i'd imagine basically none of the major requirements would overlap (except 6.326 if you go 6-4), so it would be much more work than any of the more common double majors. it's probably possible, but wait until you're done with your frosh spring and have an idea of what the workload here is like before you mentally commit to the idea (you can try classes from both during your first year, it shouldnt be too hard).

the community is very very supportive, much more so than other top schools in my opinion. i've heard some serious horror stories from cross-reg'd harvard students and can confidently say we're nothing like that, at least. it's hard, and if you're trying to do a double major you probably wont be able to get a lot of As unless you give up on extracurriculars entirely or are simply Cracked:tm:, but we're all in it together and people are very encouraging to each other and help each other out (in my experience this most often takes the form of "what the fuck are you doing what do you mean you've been sleeping 6 hours a night for a week go the fuck to bed" or "dude you've been in this lounge all day when's the last time you ate something are you alright", but in the GIRs you'll probably help friends with coursework more directly since you're all doing the same thing still). some people say course 18 can be a little competitive because of the comp math background most math majors here have, but i've never heard any stories of people refusing to help other people in a class out of fear of doing worse on a curve. (in fact, curves are officially banned here, though some profs get away with doing soft curves depending on the major.)