Hello, @Ahmed,
1. Yes, it is Ok to use compatible plugins. Our Theme XStore is Compatible with the W3 Total Cache plugin. We always recommended only using the plugins that came with the theme package OR those are compatible with our theme, here is the list: https://postimg.cc/F7Zxcb8L
2. A child theme in WordPress is a sub-theme that inherits all the functionality, features, and style of its parent theme. Child themes are a safe way to modify a WordPress theme without actually making any changes to the parent theme’s files.
To ensure you’re designing for the user experience, you might need to create a child theme. A child theme allows you to modify the appearance of your WordPress website without editing your theme files directly.
No need to choose the child theme as the main/parent theme. They are not always required. Remember, they are only required if need to modify theme code and/or files.
Changes in the parent theme will be lost after the update, so add your custom code in the style.css and function.php file of the child theme. You can read furthermore here: https://xstore.helpscoutdocs.com/article/32-child-theme
Regards 8Themes Team.