Will it work out-of-the box with ISPConfig or do I have to do something?
It works out of the box when it is installed before ISPConfig gets installed (see perfect setup guides). If you install it later, you will have to run a manual ispconfig update and choose to reconfigure services after you installed pure-ftpd. But you will have to do that anyway as the guide you followed above broke ispconfig and ispconfig has to be repaired by a update run that removes the modified files.
There might be other config steps nescessary on your system depending on the linux distribution, so you should really read the perfect setup tuotrial for the linux distribution that you use befor you install pure-ftpd.