Crocus could not have been Gol's crew doctor if we assume the story he tells about him is right. 50 years he was already the guardian of twin's cape as the "laboon's pirates" came into the Grand Line. When they left, they asked Crocus to take care of the whale while they would be sailing arround the world, which he did, 50 years long. So I think it's a safe bet to assume Crocus did not left Twin's cape during the leadt 50 years. It definitely makes it hard for him to be Rogers doctor in this conditions. Though his talking about Rogers are intringuing, so is the monologue of Dr Kureha as Luffy leaves drum.
Of course, as Cricket said about the Sky Island, until proved wrong everything is possible on Grand Line. I guess this one main principle to Oda's story anyway. So though I will noty sate that Dr Kureha was Roger's doctor, I still like this option more than Crocus.
As for Zeff, he did have his crew as he sailed Grand Line (thus during one year according to Krieg). Still the fact he saved Sanji back then, shows that it was way after Gol's execution (as Sanji who is now about 20 was then 10). So the fact he had his onw crew does not exclude he could have been part of another one (for instance Shanks and Buggy used to be on the same crew, hypotecally Whitebeard's, pure speculation there though but they both seems to know him quite well) and though I know nothing even suggest him beeing Roger's cook, I like the idea as along with Kurea they would definitely be a good basis of the deadly crew which I'm sure Roger had!
I also like the fact that they're both big dreamers (Zeff is one of the few adult believing in All Blue and Kureha believes she can save anyone) would perfectly fit with a D.'s crew.
So yes once again pure speculation. But man, do I like the idea.