No, the Welsh did not wear kilts. The Cornish did not, the Bretons did not, the Manx did not, the Irish did not, and for that matter, neither did most of Scotland.

Though recognized today as the national dress of Scotland, it must be remembered that it was the dress of the SCOTTISH HIGHLANDER, not Lowlanders.

It is a nice idea, however, to consider the kilt as the evolution of Gaelic clothing. In some ways it is. It serves to reinforce the connections between Scottish and Irish culture, which were once very close. Celtic solidarity as a whole is a laudable ideal. Promoting similarities is always better than pointing out differences.

As for the Pictish being Germanic, this is the first I've heard of that theory. And I took Celtic Studies! The fact is, we don't know what the Picts were. Some speculate that they were Brythonic Celtic (akin to Welsh, etc.) which would certainly explain their quick assimilation into Gaelic culture under Coinneach Mac Ailpein. Others think they were indigenous, perhaps descendants of the Megalithic stone builders. They even lived in Ulster where they were called Cruithne. Who knows?