I am wondering, having my system currently installed with XAMPP, is it okay if I will install another Apache, PHP, mySQL (separate installers)?
you CAN but it's not recommended. because they use same port (e.g 80,443, ...) and If one of them is working , the other won't work or worse it's possible to send data to wrong program. e.g instead of sending data to original Mysql, it's possible to send them to XAMPP's mysql. As my experience, doing such thing just confuses you, so it's better to use only one of them and uninstall the other one.