I am semi-retired and use several Adobe apps - mostly Photoshop and InDesign running on a 2015 MacBook Pro Mac OS 10.11.6 - El Capitan. I want to upgrade my MacBook with a 1 TB SSD, but the SSD comes with Mac OS 10.13 - High Sierra.
Questions - - -
Does CS 6.5 run on High Sierra?
Will Adobe support me in reinstalling that legacy software version 6.5?
If you want to keep your museum software running as long as possible, don't mess with the system. Especially don't upgrade macOS. If you want to run later stuff, consider dual boot. There is, by the way, no CS 6.5. Maybe it was 5.5.