In most cases, you are required to take TOEFL or IELTS. However, if you proved that you are full time student for last four year in USA, some universities accept you without TOEFL( example, university of waterloo)
Foe GRE, it depends. I know University of Guelf does not required GRE for certain program in Msc. And chances of geting TA/GA is competitive. But again some program, there are fewer Grad student and easier to get GA. If you are PR, you can get OSAP too.
The best person to ask GA/TA would be the professor. If you email them directly, they will reply or simply call them.
I live in Guelph, so I have knowledge about this area. Again, I am not an expert but I can ask Department head if that helps.