As for taking pictures of random people:
Pretty much as long as you ask first and they aren't obviously busy with something else, generally it's fine to ask. Just sitting and chatting with friends is usually okay. Talking on the phone, eating, crying (heaven forbid someone asks a picture when someone is crying but I've heard stories...), sitting with parts of the costume removed (thinking of larger armor builds or fursuits or the like more than sitting with their shoes off or something) or while repairing part of a costume, etc., is usually seen as rude. If they are just sitting with costume intact to take a short break, the worst they can do is say no. (And if you see someone you want to photograph who is otherwise occupied, make your rounds and come back and see if they're available.)
Fanime is a much better con for taking pictures than SacAnime, anyway. Sac doesn't really have an area where cosplayers can congregate and stand around and wait for photographers to take their picture, and photographers don't really have a place to stand around and grab cosplayers. (Some people use that second floor of the convention center, but even then, it's mostly a hangout area and walkway.) Fanime, on the other hand, has the entire area outside the entrance of the convention center open to you. Since it's a wide-open area with lots of little nooks and crannies and lots of people milling around, if you set up shop here, you won't be blocking anything, and there's tons of areas you can photograph in (especially if you like blank walls as backgrounds). There are some gatherings that take place in this area, but otherwise, cosplayers often end up hanging around underneath the sculpture, near the stairs that lead to the expansion, around the planters/bark area, or up near the convention center itself along the many walls. Pretty much anyone hanging out in this area is going to be fair game for photography, since that's why most cosplayers end up hanging around this area -- there aren't a lot of really good sitting areas around there for long-term hangouts with friends, and it's busy enough with people walking around, taking pictures, and entering/exiting the convention center that most people don't stand around there while they're doing something else (again, the caveat of "unless they are").
For taking the photos themselves, a lot of photographers will take cosplayers aside to a slightly more secluded area and do a little mini photoshoot of several poses. You can also take a few quicker snaps of cosplayers where they are. I've also had photographers who didn't have much else going on relocate to another area for an on the spot shoot (such as walking over to the Fairmont for a different backdrop). You can do this type of shoot in addition to actual pre-scheduled shoots.
Be sure to have cards with your website/FB/wherever the photos will be posted, and hand them out whenever you take a picture.
I think gatherings have already been covered.
As for scheduling shoots, I'd have to see a profile of your work and figure out my own lineup first, but I'd be willing to if your style matches up with any of my costumes. :]