A lot of course will depend on your ambition level and budget. Personally think your requirement for discs and two wheel size compatibility will drive up complexity. If you keep it simple with a proper 26" wheel frame, cantis, mechanical brifters, up to a 10 speed cassette, two chainrings, you can be up and running with something relatively lightweight and nimble.
When you go beyond that with discs, you are getting into more modern hydros or mechanical disk brakes, 11 speed and it would beckon the question would it not be best to find a modern small wheel gravel bike straight away with a new GRX gruppo?
Another potential retro brand to look out for would be Sunn. They did a large range of full rigids in XS even into 2000s.
When you go beyond that with discs, you are getting into more modern hydros or mechanical disk brakes, 11 speed and it would beckon the question would it not be best to find a modern small wheel gravel bike straight away with a new GRX gruppo?
Another potential retro brand to look out for would be Sunn. They did a large range of full rigids in XS even into 2000s.