Yes, you can safely have Paratext 7 and Publishing Assistant 5 together with Paratext 8 and Publishing Assistant 6.
Yes. Simple style templates (.isty) and template bundles (.tb) created in PA 5 can be used in PA 6. In the case of template bundles be sure that you use the same or later version of Adobe InDesign that the bundle was created with. In PA 5 these files are stored in your My Paratext Projects folder. You will need to copy them to _PA_StylesAndTemplates in your My Paratext 8 Projects folder in order to use them in PA 6.
Publishing Assistant is fully tested and supported for InDesign version CS6, CC, CC 2014, CC 2015, CC 2017, CC 2018 and CC 2019.
Theoretically Publishing Assistant also can work with InDesign CS, CS2, CS3, CS4, CS5, and CS5.5. However, only limited testing has been performed on these versions. And only limited support is available for InDesign versions prior to ID CS 6.
The answer to this question depends on the project that you are about to typeset. If the project is in Paratext 8, or plans to migrate soon, then you need to migrate to PA 6. If the project plans to stay in Paratext 7 until the typesetting is done, then you need to stay with PA 5 and should consider upgrading to PA 5.1. Read PA upgrade and migration FAQ and Paratext 8 FAQ. Note that PA 5 and PA 6 can safely coexist on your computer and will allow you to typeset both Paratext 7 and Paratext 8 projects respectively.
No. Your Paratext 8 registration record will keep your Publishing Assistant status. To check it log in to Paratext Registry and click on name in the top right corner. Select Registration menu item. You’ll see something like this:
Third line will tell you whether or not you are authorized to use Publishing Assistant.
If you think that your PA status is incorrect please contact support.
Long answer: You can keep using PA 5 (or PA 5.1) as long as your computer, current Windows version, current Adobe InDesign version and Paratext 7 keep running. Please keep in mind that Send/Receive servers for Paratext 7 will eventually be shut down and thus getting project updates might become more difficult. If the translation team moves to Paratext 8 you will need to use PA 6 to typeset their project.
Short answer: We currently intend to continue supporting PA 5.1 until at least mid-2019.
In terms of typesetting features PA 5.1 and PA 6.0 are identical.
The major difference is that PA 5.1 work only with Paratext 7 translation projects and does not work with Paratext 8 projects. Conversely, PA 6 works only with PT 8 project and is useless for PT 7 projects. Please keep also in mind that PA 6 requires separate registration.
PT 8 and PA 6 is a totally different universe from PT 7 and PA 5. They do not communicate officially. You can safely keep both sets on your computer for a while if you need to.
Check with the translation team of the project you about to typeset. If the project migrates to PT 8 you need to go for PA 6 in order to help the team. If the project stays with PT 7 there is no urgent need for you to migrate.