Another Canadian! Yay. :)
That is a fine set-up, I'd reccomend keeping 5-6 cories, since the pygmy cories are so tiny they prefer some more friends to make them feel safe. It is not overstocked, considering you have a 20 gallon filter.
Really a wildcard, you can't really know how they will react until you try it. I would reccomend you put the amanos in first, as if he won't tolerate them he definately won't tolerate the cories.
Live plants are completely optional. I have 3 tanks, one being a betta/nerite snail and two community tanks. One of the community tanks is a natural planted tank, meaning it has plants planted in soil, the other community tank has a bit of floating hornwort since it was growing too quickly for the other smaller community tank. The betta has no live plants, just silk, but I do plan on adding some of the hornwort since it grows so dang quickly. haha Only have live plants if you want the extra challenge, and your wallet can handle it. I'd reccomend some basic floating hornwort, as it is insanely hardy (actually an invasive species, impossible to kill), and about 3 dollars at petsmart for a small bundle which will quickly turn into a large bundle. Also, whenever you get live plants you will get some free pond snails along with it. These little guys don't poop much at all and stay tiny.
I hope I answered for questions well! :)
Also, make sure your tank is cycled.
10 Gallon Tank:
~Countless Pond Snails~1 Espe's Rasbora~1 Harlequin Rasbora~
5 and 5.5 Gallon Tanks:
Avatar by Fenghuang
Many of my scaly friends from my original tank from 2011 have passed, except for two! I am in the process of re-doing my tank. :)