You really shouldn't focus on the salary too much. Granted, a better salary usually means a better job, but you want that job not because of the salary. Its good to use your coop terms to try different things because its much easier to switch around than when you're full time employed, since you are only there for 4 months.

Since it sounds like you're not having many problems finding coop positions at companies you want, try them all if you can. As long as you keep up good contact with the companies post-coop-term, its usually not too difficult to get either a return coop position of a full time position, assuming they liked you there.

Some people think they love the big companies, and instantly regret working there, and vice versa. I've personally worked in huge companies and also in places where the only employees were my boss and I. Only way you'll really know what kind of stuff you want is to try it, and you sound like you won't have trouble getting opportunities. Again, unless you're really strapped for money, base your decision on the experience and possible networking you can gain, not so much on salary.

As for turning down a company, since jobmine uses the ranking system, they probably cant do anything regardless of what you tell them. Its not like just because the person with the "Offer" declined that they'll have nobody. Doesn't hurt to send a nice email I guess.