older time capsule compatibility?

In addition to Bob's comments, if your data is critical to you, you may want to consider a multi-tiered backup strategy that includes both incremental and clone backups of both of your Macs. Relying on both types will save you a lot of grief down the road should either of your Macs experience a hard drive crash.

FWIW, we see a number of issues using Time Machine with the Time Capsule, especially with the latest incarnation of macOS. So just be aware that using this approach alone may not be the best approach to follow in the long run. If you prefer to continue using Time Machine, either use a directly connected external drive or a NAS that supports Time Machine. One example of the latter would be just about any NAS from Synology.

An A1254 version of the Time Capsule is at least 9 years old, and probably older. It is classified as "Obsolete" by Apple, so this product would not be recommended for any type of task involving the safety of your data.

Your Mac running 10.6.8 is compatible with the current Time Capsule if you want to back up the Mac using Time Machine. But, you will need to use the Mac running 10.11.6 to set up and administer the current Time Capsule.

Please keep in mind that the "current" Time Capsule was introduced almost 5 years ago, and Apple has make no improvements to the product since it was introduced. The product is years overdue for an update.

Backups of your Macs would occur much more quickly and a lot more reliably if you simply connect a USB drive directly to each Mac.