Sushi-

If you're worried about the tartan police don't be. They don't exist. The lines about universal tartans, clan tartans, and the "right " to wear a tartan are fuzzy. With little exception anyone may wear any tartan. There is no law anywhere preventing it. If your eye has caught a tartan which is restricted you'll not find a kilt maker who can get the tartan to make it.

Many people like to wear a tartan to which they've attached some significance; clan, county, occupation, state, military branch,etc. If you feel this would make you more comfortable in a kilt then by all means decide what significance works for you. However, there isn't anything wrong with approaching the tartan as fashion either. Being in Japan I'm sure you're seeing every spectum of the fashion scale. Complete fashion disasters included.

It sounds to me like you'd like to have a connection to your tartan, and I'm sure you'll be able to find a tartan with some significance. If you feel a connection to the Stewart go for it, but keep the entire Stewart range in mind. As the lines are fuzzy you can have the best of both worlds by wearing a Stewart tartan to which you feel connected, and like at the same time. The Black Stewart being an example.

As for writing to a clan chief; not needed. They haven't the authority to tell you no in the first place, and it'd be uncomfortable if you'd had you're heart set on a tartan, and the reply was " we'd rather you didn't.". The alternative, however is to find a clan association to which ever tartan you choose, and get from them a list of highland events they'll be at. Then take the time to meet them and let them know how much you like their tartan. Connection or not, they'll appreciate the gesture.

Good luck.