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I'm pretty much just transcribing the Bulbapedia page, here. When you rematch her in HGSS, she has a Girafarig, Lickilicky, Bibarel, Delcatty, Clefable, and Miltank.
In B2W2, in the Johto Leaders Tournament, she uses Miltank, Wigglytuff, Tauros, Ambipom, Ursaring, and Clefable.
In the Type Expert Tourney, she uses almost the same team, with Wigglytuff and Clefable swapped for Blissey and Lopunny.
During the Challenge the Champion Lance! bit, she uses a Watchog, Cinccino, and Miltank.
She has teams in Pokemon Stadium 2, but they aren't purely monotype. Round 1 is Miltank, Clefable, Wigglytuff, Gloom, Dodrio, and Stantler. Round 2 is Miltank, Persian, Golduck, Mr. Mime, Tauros, and Donphan. She also uses a Nidorina in the anime and one of the manga. In another manga, she has a Miltank, Cleffa, Igglybuff, and Smeargle, while in yet another she has a Miltank, Cleffa, Ditto, and Magby.
If we want to be sticklers, Clefable is out. There's something of a precedent for her not using only normal types, and indeed, we can just swap Clefable out for Wigglytuff. The Clefairy line isn't strictly required, as there are instances where she doesn't use them. Conversely, Miltank is a necessity. It's her ace, appearing in every team she's had.
If we're going for type diversity in a monotype, we'd probably go with Miltank/Wigglytuff/Girafarig/Bibarel/Dodrio/Mega Lopunny. Dodrio could feasibly be swapped out for pretty much any other Normal/Flying Pokemon, but she's appeared with a Dodrio before, so it's chosen by default.
Personally, though, I think that's the wrong way to go about it. Given Whitney's character, she doesn't seem all too concerned with typing. As it says for the Magby section, "Whitney owns a Pokémon that is not her specialty type, Magby. However, she still thinks that it is cute due to it being a baby Pokémon." So we're looking for cuteness, here. In that respect, we'd probably get something more like Miltank/Wigglytuff/Delcatty/Blissey/Cinccino/Mega Lopunny.
