Is it appropriate to sand a hardwood floor with a 20, 60, 100 grit sequence?
I've only refinished two hardwood floors myself but I do know a bit about woodworking given my dozen years of experience as a cabinet maker. The first floor was done with a rental rotary sander. This violates one of the cardinal rules of wood sanding: always sand with the grain. A rotary sander will leave swirl marks and you'll have to spend a lot of time with increasingly finer grits to remove all of the swirl marks. I started with 36 grit which was necessary due to some staining of the oak and progressed through 120 grit in roughly 20 grit increments. This gets expensive fast! The second floor was done with a belt sander like the pros use. You sand with the grain so overall you have less sanding to do since you don't have to remove swirl marks. There are two major caveats with a drum/belt sander. First, the ones from rental places are usually way out of adjustment. Which leads to the second caveat, it really helps to know what you are doing, both for the adjusting and the sanding. Otherwise you end up with lap marks where there are seams between the adjacent passes. Another issue is ending up with a washboard effect. Tread cautiously! There might be some YouTube vids on how to use a drum/belt sander.
To answer your question directly, don't use anything courser that 36 grit and in fact given how good this floor looks you could probably start with 50 or 60 grit. Good luck!