I tried enabling limited tourism in my summer town. I disabled all other worlds except one which is a custom town. Despite that, sims from China and France showed up even though those worlds are disabled.

I haven't seen this for myself yet but, to be fair, I usually keep toursim on/off for all worlds without getting too selective about them. But just a shot in the dark here, is it all possible that the game clock was running in between arrival and/or allowing tourism and the disabling to the extent that it's just the first wave of tourists that came from the now disabled worlds, or has it been more persistent than that?

Have you tried disabling tourism, keeping the worlds disabled, forcing the ones already there to go home and trying again with no tourists in place at the time? Temporarily, NRaas > Register > Tourists > Chance of Leaving > 100 and again temporarily NRaas > Register > Tourists > Allow Tourists > False then "reboot" the tourism with a smaller Chance of Leaving again after all have cleared the scene.

I normally keep tourism totally off but this world is very small (~20 lots total) so I thought letting 4 in would be okay. Also its a beach vacation world.

I tried several combinations. I always started from a save where Tourism was already disabled and there were no tourists. I even went so far as to set all worlds as disabled under Disabled Worlds and then turn on Tourism anyway and here comes Egypt/China/France, but only those. This save has about 10 custom worlds in it in addition to China and Egypt. I never went to France so I don't have an NHD of it. I never get tourists from any of the custom worlds with them disabled and I actually never got any from the single one I enabled once because the 4 slots were quickly taken by World Adventures world tourist sims.

Thanks for your advice, though.