Big Mom is not going to be a part of their fleet, that's an even worse idea than a theoretical truce. Nothing about her characterization thus far (Like bro, she's a cannibal that destroys entire countries over sweets) should lead anyone to believe this. This includes her willingly letting the Straw Hats go after being fed the cake, but at least with that, you can understand the slim possibility of it being so good that Big Mom forgives and forgets. But for her to make an alliance as well? Hell no. And that's not even including the unfinished business Luffy and her have regarding Fishman Island. We know more about Big Mom, yes, but she is up there with Caesar in terms of being virtually irredeemable.
And speaking of those that feel that Big Mom will let it go, I mean, Jinbe fed her that cake when she was in one of her hunger pangs, but still refused to let him leave her forces without spinning that rigged roulette wheel.
The best outcome for her characterization so far, and frankly, the impression of the arc overall, is to just have her knocked out by the cake (Like Sanji said he literally would do), and then have the Straw Hats escape immediately afterwards while she is unconscious.