duncan,
welcome! I think if you peruse the pictures that have been uploaded, you'll find that many of us are very comfortable wearing a t-shirt or other casual wear with our kilts. For the best guide for all things pleated, check out Hamish's photo gallery. Hamish is truly a kilted legend, and is the standard by which many of us gauge our kilt wearing. I hope your kilted journey is long and fruitful!
~Sav