I am currently running 26 tires on my road bike. I am considering bumping up to 28s for my tour. I have a bunch of 26 tubes. I have read that it is actually a good idea to use a smaller tube to prevent pinches. Is using a 26 tube in a 28 tire too much of a stretch. Pun intended.
I have 28 and use 18-23 tubes all the time. I rarely have flats, the last one was due to the tire wearing out and a piece of glass getting embedded in the tire. I am also assuming you are talking about 700c tires. Plus the tubes being smaller make them easier to install IMO.
I run the skinnier road tubes in the wider touring tires all the time without any problems. Must flats are caused by wide tubes getting pinched-- skinny tubes are easier to install. Plus, many touring tires that are labled 28-35 mm have so much rubber and belts in them that they are close to 25-26 mm inside anyway.