Barrel
I live in Northern New York (just a few hours from Toronto) and I have an outdoor barrel, too.
I've had good luck with guppies, platies, and the American flagfish (they're a type of killifish and have been mentioned already, as well). The livebearers breed like nuts. (I only had female flags.) I don't have any problems with mosquito larvae.
This summer I think I'm going to try swordtails and possibly some of the 3-4" barbs. Some barbs are really neat, they look kind of like goldfish because they're related, but stay a lot smaller.
I plant my barrel, too.
To buffer the pH you could try a coral substrate on the bottom of the barrel, like they use in African cichlid and saltwater tanks.
Good luck, have fun! I love my barrel.