As mentioned earlier, on campus jobs are extremely competitive and hard to get a good one (like library and stuffs). But if the university has a big campus (aka public university), there would be loads of food shops and they constantly hire people and they pay at least the minimum wage set by the province (8 to 10). Better on campus jobs like bookstores and libraries are more of "referred" basis and they take people whom the admin knows. If you can get in touch with managers/executive body members of some sort of associations, that would be a plus for securing better job. Like here at uofc, they pay $15/hour at library while $10 (at max) in food shops.
About the 20hours/week, its just a guideline and nothing mandatory. If you can manage, you can work for more than that. But just a heads up, the study over here literally "kills" you if you don't give it enough time. I once enjoyed a long weekend without studying and had to go through a tidal wave!
Those who arrive here at Fall gets advantage of working full time during summer but for people coming in January, that's a negative point since the work permit would be issued at the end of summer for them.
Over here, work and education is more or less like a trade off of each other.
.
.