It isn't hard to see that Luffy and Kid will eventually end up joining forces in the fight against Kaido. Although their alliance will likely be temporary, it'll still be something that will play a massive role in taking down Yonko Kaido, just like Luffy's alliance with Trafalgar Law.

Is Eustass Kid an enemy of Luffy?

Eustass "Captain" Kid is the Captain of the Kid Pirates. He is an unfriendly rival of Monkey D. Luffy, Trafalgar D. Water Law, Roronoa Zoro and all the Worst Generation (except for his right hand man, Killer).

What episode does Kid ally with Luffy?

"Formed! The Luffy-Law Pirate Alliance!" is the 594th episode of the One Piece anime.