So, I've made progress. I'm rebuilding the rear wheel with a Deore 36h hub so that I can upgrade to a 9 speed drivetrain. When I had the wheel apart, I put the calipers to it and found the inner width was 19.3mm, plenty small enough for a 28mm or larger tire (according to Sheldon Brown).

My initial assumption is to go as narrow, light and as high pressure (100psi) as I reasonably can.

• Is there any reason to consider lower pressure tires? The soft, squishy steel frame of the triplet absorbs most bumps, so I don't know how much absorbtion the tires need to do. (But, I will listen to anyone with experience)
• Any disadvantages of 28mm or 32mm tires, opposed to a 35mm (1.5") tire?
• Are "Tandem" rated tires necessary, or should I be fine with any reasonable 26x1.5ish tire? I'll probably only use this set of tires for 6-700 miles, because I can only use them when I'm driving the bike and don't need to drive the gravel on road.

BTW, the tire I'm currently leaning towards is the Serfas Seca 26. I'm new enough to this that I'd love to hear if there is anything bad about this tire that should sway me against it.