Would either of the Yaki or Hermit tours (or both) be manageable by seniors in relatively good health? It doesn't look like either has any steep sustained climbs?
Either one would work for you - but we recommend the Hermit Tour. There is one small section of incline (less than a quarter mile), which people who don't feel capable of riding can walk their bikes up. We have all ages do this tour every day.
Keep in mind that almost the entirety of the Rim Trail is closed to bikes. On Hermit Road you'll be on the road with the shuttle buses out of sight of the canyon most of he time except at the viewing points where the buses stop. The last mile of the Rim Trail before Hermit's Rest is paved and open to bikes but away from the canyon with no views. If you just want to ride a bike then go for it but if you want to see the canyon, forget the bikes and hike the trail.
Get quick answers from Bright Angel Bike Rentals and Tours staff and past visitors.