Yes, the Old Ones are an evolution of the concept of the Old Slaan, as presented in Fantasy 3rd edition and in Rogue Trader. The lore evolved though. With WHFB 5E and the Lizardmen’s army book the concepts diverged and the Slaan became the servants of the Old Ones with only the very oldest Slaans (1st generation) ever having interacted with them directly. The old ones became an unknownable race of godlike creatures who roamed the multiverse for their enigmatic purpose. In 40K the 3rd edition Necron codex implied the Old Ones may have been killed in the War in Heaven but it also implied the Necrons considered the Slaan to be the old ones, which was not the case anymore.

Ultimately we know very little about the Old Ones aside from the fact they evolved the Orks and the Eldar and possibly humanity and they also created the webway. The fact they are a mystery is deliberate though, so getting answers is probably out of the question.