After reading the Zamus page on Dojo, I reckon that to get Zamus you probably just have to complete some part of the 1P mode as Samus. These quotes from the site convinced me;
'This isn't a pure character addition.' - Nah, it's a character already in the game but in a different form.
''remove'' - She won't actually remove her suit at any point. Zamus is just how Samus would function without the power suit.
'And don't worry—you'll still be able to play as standard Samus, too.' - This got me convinced that they're different. Sure, it doesn't prove it, but it suggests to me that using Zamus doesn't affect your ability to use Samus in any way. Some of the poll answers make it seem that you're going to have to mess about and not have much freedom in which one you play as, like probably having to have items on with the Super Smash method, which would mean no choice if you play without items, or you're stuck with a character you don't want to play as for a bit until you get the item to change to the one you want, which can just be defended by your opponent to stop you, limiting you to a character you're either not as good with or just don't want to play as.