Interesting that mixing focal lengths or color and mono is frowned upon. I can understand why but hadn't thought of it before.
I think he says:
"I know lots of street photographers who use all different focal lengths (21mm, 28mm, 35mm, 50mm, 85mm) and so forth."
And
"There is nothing wrong about using different focal lengths in street photography, but personally (when looking at other peoples work) it is frustrating to look at."
If there's a problem, he is the problem
Jay Maisel walks around the streets with his D3s and a 28-300 zoom lens.
Here's a Street Photography e-book written by another photographer that you can download:
I think one common thing about street photography is: it's about people (but people don't have to be in the photos to work).
One more thing. If you shoot in the streets in Quebec, anybody who appears in your shots can sue you (not just because you're going to use your photographs for commercial purposes).