Hi there Jesse,
Thanks for getting in touch with us 🙂
As you say it’s highly advised to match major versions of the plugins. By major versions I’m referring to the first two numbers (4.2). The third number refers to the current maintenance release (4.2.1) and it’s not the same for all plugins. Finally in some cases you’ll see a fourth number (4.2.7.1) which indicates a hotfix release to squash a security related issue or any other high impact issue that cannot wait until the next maintenance release is out. You can find more details on this in our release schedule page.
So rounding up, the first two number need to be the same for all plugins (4.2.x). And it’s ok to use The Events Calendar 4.2.7 with Events Calendar PRO 4.2.6 🙂
Please let me know if this answers your question,
Best,
Nico