I think Usopp and Nami will probably take the weakest members which seems to be Page one and Ulti, at least the way the others are speaking to them
Usopp vs Page One
Nami vs Ulti
then knowing oda he'll have both Nami and Robin fighting girls
Robin vs Black Maria
so that leaves Franky, Brook and Chopper and Who's-Who, Sasaki and X Drake
but like you said we don't know if Oda will go strictly Straw Hats vs Beast Pirates, who the hell knows how the Big Mom's Pirates will fit in
I thinks X-Drake will be fighting Who's-Who when he reveals his allegiance. I judge Who's-Who and X-Drake to be the two top dogs in the Tobi Roppo for the following criteria: they are men and their body build. They are also seated in the center on the panel.
And about Brook… I think he might fight Apoo, because of their similar fighting style. The fight might not look that cool though, but it has to happen I think.