I may hate this idea because I shared a bathroom with my disgusting N/GC brother

Do you know how the on-campus housing floor plans are set up? I ask because if it's anything like mine, allow me to set one very realistic expectation: any and all bathrooms will be nasty in college, especially the dorms. My college was set up so there were 11 ladies sharing a bathroom (two showers, two sinks, two toilets) in the dorms. No, the people in singles did not get their own bathrooms. Believe you me; the mess these ladies can leave will not compare to your brother's. It was a bit better if a student lucked out on an apartment on campus where the bathroom was shared among only 6 ladies. And if you're very lucky, your mates will agree to abide by whatever basic rules you guys set up at the beginning (pick up hair after shower; wrap feminine products; etc.).

If you really want your own bathroom, you're probably going to have to search for off-campus housing, because having a single means you have just that. A single bedroom. You still have to share the bathroom.

I am worried that she’ll be either a freaking girly girl social butterfly (kind of the exact opposite of me)

Generally, there are little questionnaires for everyone to fill out asking how you feel about overnight visitors, smoking habits, preferred study environment, bedtime schedule, etc. The colleges will then try to match accordingly. Of course, it's not a perfect process, but beggars can't be choosers. If you find your roommate and you absolutely cannot get along, you can request to be moved elsewhere the next quarter/semester (but no guarantees on what your new roommate would be like either).

I was in a triple my first year--one of whom liked to party and the other who was more studious. We agreed on some ground rules, and it all worked out. Parties happened elsewhere, and conversations were not initiated when our respective headphones were on. Point I'm trying to make: it doesn't really matter if the roommate is the opposite of you in personality. You're really just sharing a space to sleep (and maybe study) in--not being forced to be joined at the hips 24/7 with her.

Although, do be aware that you'd have to deal with their quirks. Perhaps they like to throw their dirty laundry on the ground while you like the floor clean. Do they like to sleep late and you early? Are either of you a light sleeper? Do you like to eat unholy amounts of gas station burritos and let off those dying whispers in the middle of the night? If you don't want to deal with that, you might want to look at getting a single. And you could still interact with the others (your neighbors) on the floor if you get lonely.

put out flyers around campus to find a roommate I like

There most likely is a facebook page for your college where students try to find housing. Also, friends' of friends' are great because you tend to have similar interests. Or go to the page of a club/organization/fellowship on campus that you're involved in or like and ask if anyone's looking for a roommate. Might be easier to find like-minded people there.