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If anything, you should use clips from the opening versions and not episode clips. Just make sure that they work with the song you use. Note that the song shouldn't be one of the songs thst were already used on any version of Pokémon if you want it original. All of this is pretty obvious advice though.
You could make a mix of the craziest moments of the anime's 1st season, such as Gastly transforming into a mongoose and a Venustoise, Jessibelle dominating James in "Holly Matrimony", Brock punching Misty in episode 11 and don't forget: the ridiculous pokemon outfits of Salon Rockee. You could make a chipmunk version of the 1st season opening theme to theme your opening video.