I suggest not using both of them on one OJS instance until the next release of the Old Gregg theme plugin. Take a look at this post and let me know if you have any questions.
BTW the master branch of the JATS Parser Plugin now is compatible with the Default theme but it’s not tested yet. If you’d like to test, you can download it with git (I’d avoid production use for now):
git clone --recurse-submodules https://github.com/Vitaliy-1/JATSParserPlugin.git