I'm a doctoral student at McGill (Faculty of Law), so hopefully can help clarify some things here.
First off, unless your program is odd, you won't have to pay tuition for the summer term. Technically you won't be enrolled over summer, but you'll continue to receive all the benefits of being a student (library privileges, etc.). The main impact this will have on you is that a lot of the major university awards are set up so that they have to paid out while you are enrolled in classes, so your $25k for the first year (after tuition) will be paid out over eight months (September to April), and you'll have to save up or make alternative arrangements for the summer. While you'll probably want to research/write over the summer, it's my understanding that the funding-based restrictions on external work don't apply during May to August.
Second, depending on what area you are in, I wouldn't put too much hope into external awards. There is a lot of external funding available for Canadian grad students, but as an American, you won't be eligible for most of it. In my case, I was eligible for two major awards—the Quebec provincial scholarship for international students (PBEEE) and the Vanier CGS. Since you're not from one of the countries with their own quotas (e.g., Belgian Wallonia), you'll end up in the general category for the PBEEE, which is ridiculous competitive. Basically, each university in Quebec can nominate four people, and ultimately a random number between five and twenty will get awards, depending on how much extra cash the Ministry of Education has on hand. I didn't even make it past the McGill stage when I applied.
The Vanier, on the other hand, is one of the most prestigious awards in Canada—$50k per year for three years, non-taxable. Each university has a quota of nominations, assigned over a three year period. (In my case, 36 SSHRC nominations over 2009-2011, but only 10 were left by 2011.) Then, it goes to a national competition where roughly 25% of nominees get awards (almost all Canadian nominees end up with other awards instead, but as international students, we're not eligible for them). This past year, the SSHRC competition (which is probably where you would be as a Sociologist) was the most competitive inside McGill, as it was oversubscribed, while they nominated practically all applicants for Vaniers through CIHR (biology and health sciences), since it had been undersubscribed in prior years. The number of nominations will be reworked for next year/year following, so hopefully McGill will no longer have the complete dearth of SSHRC nominations, but there's no real guarantee.
As for tuition waivers, there used to be a program where they waived international fees for doctoral students, but that was abolished the term before I started. It sounds like your department has been able to reallocate resources, but it initially seriously messed up my own funding situation, since I was admitted right at the changeover.