No big deal on the out-of-level issue
I used composite shims that don't compress over time or have problems with moisture
I bought mine at Home Depot. And 'cause they're brown you may not see them with your floors
I can't find a Canadian link for you, but
these are what I bought in the door department in Burnaby
They're just as easy to score and snap as the cedar ones too
Just to touch on your electrical ...
If you run BX, are you planning on it being in-wall just to the receptacle ?
If not, I'd suggest you go with something more water-compatible rather than a metal jacket that can corrode around your tank