Well, you can get Jigglypuff (who is also on Whitney's team iirc) in Johto, so use her, switching is permitted after you get to Kanto. I assume you're playing Crystal since Clefairy is only found in the Odd Egg in Johto, which is Crystal only. Jigglypuff isn't found till Goldenrod either in Crystal, but Slowpoke is found before the second gym, so you can't hack anyone in early, although you can opt not to use Slowpoke (use it as an HM slave!). So use your starter until you get Cleffa, then you'll have to use Jigglypuff in Johto and switch it out for Mr. Mime later.
Does that work/make sense? I know you like combining challenges (I actually don't try to do that at all), so this is the best I can do.