Yes, it can coexist with any other Joomla (security) extension. I'm continuously implementing improvements to make it as universal and robust as possible.
There are currently some recurring false-positive issues involving Imunify360, but I'm confident they'll be resolved as well.
The new Balbooa Forms vulnerability has been included in the definitions feed since yesterday.
HTProtect's self-updates and automatic extension updates require Joomla 5.4 or later because they rely on the Joomla Task Scheduler. All other features work on Joomla 3 as well, with compatibility ranging from Joomla 2.5 to 6.x and PHP 5.6 to 8.5 (PHP 7.4+ recommended).
If you prefer to use the .htaccess Maker from Admin Tools Pro, there's also an option for that under the Protection Shield's Extended Settings.
See:

Since HTProtect is already running on more than 5,000 websites, it will continue to evolve, and I'll keep doing everything I can to provide the best possible free protection for both current and older Joomla websites for years to come.