The Johnson LMG was used by the joint Canadian-American 1st Special Service Force (Devil's Brigade) in Italy. I don't think regular Canandian soldiers ever used the LMG or rifle.
With the US Paramarines, the Johnson rifle was preferred over the Garand because of the quick change barrel. It could be reassembled in 90 seconds. But Paramarines and Raider units were pretty much nonexistent beyond 1943. All the rifles were originally meant for Holland, but the Germans took over, so the USMC took the 30,000 rifles and used most of them in the Pacific.
http://www.johnsonautomatics.com/ - This site is probably the most informative, but it still lacks a lot.
"I have to say I would take french and italian weps before either of those though."
Agreed. The Johnson weapons would be neat additions more so than another SMG, but aren't really necessary.