Those matchups probably make the most sense (although isn't there a fourth admiral too? Wonder who he would logically match up with. Maybe Franky or Usopp? Or maybe all the remaining SHs would take him on, as the above person said), but I'm not convinced the Admirals will be defeated. During Roger's time Garp was never definitively defeated, so I'm thinking the same could happen in the Strawhat's story - the admirals end up in skirmishes with the SHs, but the SHs manage to escape somehow without being dragged into full-on fights. The only exception would maybe be Akainu, although I think it might be more fitting if Sabo defeats him (maybe Luffy gets a hit in and then leaves the rest to Sabo), since he has Ace's old Devil Fruit and Luffy focuses on Blackbeard. Another reason I don't think the admirals will all be defeated is how well Luffy got on with Blind Man, since it wouldn't be very fitting for Zoro to brutalise someone Luffy gets on with. Plus Blind Man seems similar to Aokiji, who left the marines and is no longer an enemy of the SHs. I could see Fujitora leaving the marines or opposing Akainu in some way eventually (maybe alongside Aokiji and Coby, possibly even alongside pirates if some big truth is uncovered that causes some marines to fight the World Government).