From your explaination I could not take a 28" model 870 Rem & shorten the barrel to 20" myself...is that correct?
No, actually you can shorten it a little more than that even. 18.5" to be safe, and that would remain non-restricted.
Or you can just buy yourself a second barrel from Canada Ammo that's 12.5" long and still stay non-restricted, as long as you keep a full length stock on the firearm.
See, there's also other exceptions too: overall length has to meet...er...I think it's 23.5" or something?
And a semi-auto shotgun barrel that was factory made shorter than 18.5" is legal to own and use, but that changes the classification of the shotgun itself into a Restricted firearm. That means you then have to phone CFC and have it registered as a Restricted. Which means you don't take it anywhere but the range or a gunsmith, just like a handgun. And that doesn't change if you just swap between a short and long barrel, because if a cop stops you, he's looking at serial numbers, not barrel length, and if you are in the bush with a "restricted", you could be facing charges or lose your license.
Similarly, pump shotguns AFAIK are legal to have short barrels that are manufactured...but if you combine a short barrel with a pistol-grip only, (meaning no full-length buttstock), then I believe it becomes restricted.
But if you really like pistol-grip-only for shotguns, you still have an option: keep the barrel at 18.5", and you are legal and non-restricted. This is usually only an option for pump guns, as most semi-autos seem to have a recoil spring going the length of the stock.
Personally though, I think pistol-grip-only is useless, unless you are a SWAT breacher specialist, but even they likely prefer full stocks most of the time.