I agree with what DarJones mentioned about your choice overall.
Brandywine is sometimes iffy even here in Wi, let alone in Canada.
Black Krim, while it is an early tomato, is an iffy grower. It seems to have good years and off years for me. in the off years I get little if any fruit set. Even in the good years it isn't a heavy cropper for me.
I second the choice of Gregori's Altai instead of either Brandywine or Black Krim or both.
Another tomato that was quite early for me last year was Russian Apple Tree. While it has been mentioned that it's just a re-named Matina, I grew both last year and the RATs were several days earlier and enough bigger than the Matinas that it was noticable. Not a lot bigger, but enough you could see it. They also seemed to produce longer than the Matinas last year.
My own GWR suggestion would be the cherry Green Doctors as it was nice and early and had an interesting spicy taste I liked.
Te suggestion about bigger plants is also good, but when you are depending on others to do the starting, it could be a problem. Here is a page on my web site that I print and hand out at the market for those wanting those "first tomatoes".
http://knapps-fresh-vegies.netfirms.com/tips.html
It's adapted from something printed in Organic Gardening a long time ago. A neighbor of mine uses a technique similar to that with a concrete wire cage and a fence post to hold it all in place. His plants will be growing out the top of the cage by the middle of June most years. He also has tomatoes several weeks before I do.
Carol