I am wondering, having my system currently installed with XAMPP, is it okay if I will install another Apache, PHP, mySQL (separate installers)?
XAMPP is essentially self contained - the files will not interfere with the separate installations you mention (I've done it - it works fine). Typically with XAMPP, all the required files are within the single parent directory. With the separate installations, each will reside in its own folder - there should be no overlap.
Config files are also 'self-contained' (stored within the application directory in this case), so will not interfere with each other.
However, to have components of both running simultaneously, you need to change the default ports - Apache, for instance will run on port 80, by default, regardless if installed stand-alone, or as part of XAMPP - two applications cannot use the same port simultaneously.
If at any given time you are only going to be running one or the other (not both), there should be no difficulty at all (just be cautious copying configs between the two as that doesn't always work so well).