Also, I am pretty certain you can mix and match in the Sidekick. You just have to add the desired promos in like you would add any other target. At least that's what I recall reading from MigrantP somewhere. i do not own it so I cannot speak from experience.
Ronway, you are right. In hero selection, you are just given the choice of "The Sentinels" and "The Adamant Sentinels". But, if you remove them as you would a non character hero target, you can add in either version of each character.
Although a LIMITLESS amount. I have 5 Mainstays right now, and 1 Adamant Mainstay.
For what it's worth, Sentinels Sidekick has very little to do with the rules of the game, and is in no way a reference for anything.