Merry Christmas. My go-to Flash is Wally (He's probably my favorite, at least in the comics.), and I don't recall many cosmic beings that he has gone against on his own. The majority of his Rogues (Are they still bad guys? They seem to waver back and forth.) are non-powered street-level guys. And the Surfer is a major cosmic player who fights pretty much all cosmic and or magical based characters. Now, the Flash is obviously the fastest hero in comics, but would SS be able to keep up with him? I remember one comic (That was pretty good.) where he flew straight through the SUN! I think that it's just a matter of the Flash being able to take him out before he uses his power cosmic, because once he does, the fight is pretty much over. I can see Flash being all of the other Heralds (Not at the same time.), but the SS is more powerful than any of them. What if the Speed Fore is used?
Flash is of course much, much faster, at least down on Earth, maybe not when flying through the cosmos.
Surfer's cosmic senses and powers should compensate for that though and make it hard for Flash most of the time.
But if Flash over-achieves he could beat a Surfer that is not up to par. Which is pretty usual for Surfer come to think of it.