We need to know more details of the Rocks battle for me to make an accurate guess of their power-scaling because it's implied the events didn't really go like the marines said. It's possible that Roger was the one who fought Xebec for example because he was described as Roger's enemy, not Garp's. I think Xebec died as a top tier, for having the strongest pirate crew of the time. And Roger as his natural enemy might already have been a top tier as well, but not yet at his absolute pirate king level prime, which he reached later and Primebeard's equal.
I think Garp on the other hand peaked there since it's where his nickname came from and also the oldest, so might been a top tier. As for Whitebeard, he was mentioned as "young Whitebeard", so we know he wasn't in his prime, a bit like Weevil maybe who Kizaru compared to. But he probably still had higher portrayal than the rest, so i'd say at best, he was low top tier. Kaido was an apprentice, but he already seemed to be big, and as a natural born destroyer, i'd wager he was at least a mid tier. Big Mom should have at least been a high high tier herself. These are the safest bets i could make at the moment.