I dressed Rory from a very young age - practicially the instant I brought her home, but I'm imagining that they adjust fairly well. I did the same thing with a carrier, and she loves both her carriers and dressing up most of the time (the times she doesn't "enjoy" it she at least tolerates it!)
Maybe start out with a little t-shirt that doesn't have any ruffles or anything. She is more likely to forget it is on and plaaaaaaaaaay!
