Going to USA

Yes you can do so !
Fizz allows you to change your plan Coverage for the upcoming billed month. In the Fizz application go to "Plans section / Manage plans button / Adjust plan". You can then switch your plan coverage for Quebec, Canada or Canada+USA which would be your choice. After your trip is done you will be able to switch back to your previous plan to save a few bucks on long term.
Just be sure to change your plan before your trip billing month begins. As an example, if you activated your Fizz account at first on March 28 a few months ago, your account cycle (payments) happens every 28 of the month. So if your trip is between July 23 to August 1st as an example you will have to keep the Canada+USA plan for at least 2 months because your plan cycle date just happened at the middle (28) of the trip. So be sure to change your plan before it is too late since we have to do it about a month in advance for the upcoming payment cycle ! If we consider my example, the traveller should change his plan to Canada+USA before June 28 to have the right plan during his trip.
If you find out you are too late to change your plan for your trip, you can also buy various add ons from Fizz (available on the same page as the adjust plan button visible on the screenshot). The advantages of add ons is that you buy them and get the extras instantly instead of having to consider the montly cycle principle I described before. You can then buy minutes/data after choosing the country you want add ons for. It is intantaneous but more costly than simply changing you plan in advance...
Hope this helps !

Yes, nothing prevent you to switch plan every month.

Not only can you do such an arrangement, but it's technically why this option is there with Fizz. And for me it's a big part of why I decided to go with Fizz. Even if you plan to go only for vacation for a week, it might be advantageous to switch.

Cheers!