I have the following old switches in various firmware version in one Fabric.
Now I need to connect a new DS6520B switch to the fabric, I noticed that the lowest firmware supported for DS6520B is 7.1.0a I was wondering whether I can run 7.1.1 and still compatible with rest of the switches.
The Brocade Release Notes for FOS v7.1.1b have a "Fabric OS Compatibility" chart starting on page 14. Based on that chart, it appears all of your switches would be able to join a single fabric while still running the FOS versions you have listed out.
Speaking in general, a good way to determine if the FOS versions in your fabric are OK is to check the release notes of the latest FOS version (usually the version on the switch you are adding into the fabric) of the switches in that particular fabric.