If two individuals can produce fertile offspring, typically, they are considered the same species. Or at least a sub-species (ie Canis lupus and Canis lupus familiaris).

Mer and Man can interbreed. So they're of the same species.

According to Notes on Racial Phylogeny, no one knows if Khajiit and Argonian can interbreed with Man/Mer/one another. It really doesn't seem to be a reliable source though. Mostly because of this line: "http://www.uesp.net/wiki/Lore:Notes_on_Racial_Phylogenyhttp://www.uesp.net/wiki/Lore:Notes_on_Racial_Phylogeny" It makes sense that scholars would know more about them now because they're civilized. Outdated text books, ha!

Is there any evidence Khajiit and Argonians are the same species as Man and Mer?

I'd say all the intelligent races and creatures probably belong to the same genus. Species/sub-species? Dunno.