It's not the first we see a non-straw hat defeating a commander, so I wouldn't be surprised if Marco defeats King.
Also, saying that Marco is not a major character is not totally correct. He is certainly more important than Killer, Hawkins, X-Drake, the scabbards, and even Kid. Don't forget he was WB's 1st division commander and the man who fought BB. He was highly anticipated and we knew he was coming to join the alliance and fight.
Let us not forget that Marco is actually fighting King right now, so you are talking about something that may happen while I'm talking about something that is already happening, that's the difference.