Awww, I thought shrinking didn't count as a costume... like the shrink potion on engineer...
Shrinking itself, no. There's even a JP where you can get shrunk by the mechanics of the JP. In that one, your skills remain.
Tonics, or at least the ones I've used since brawling was introduced, will transform you and replace your skills. If you have several you could transform back to use a skill then take another tonic I guess.