I don`t think that there was ever a clan system in Wales comparable to the one in the Scottish Highlands, so there isn`t really a Welsh Thomas "clan." I know that Thomas is a very common surname in Wales. Until fairly recently, there was no such thing as a Welsh tartan, or a welsh kilt. (cilt) The Welsh name tartans were developed, I believe, in the 1970`s, and have slowly gained popularity. The Welsh Thomas tartan would certainly be appropriate for anyone named Thomas who is of Welsh descent.