You can add any Linux program to Plop Linux when you compile it. Some programs need additional libraries, which also have to be compiled by you.
A note about Mplayer GUI: I don't use the GUI. I do everything with the Mplayer hotkeys. Also, I start it always from the shell. The GUI will be removed in the next release.
Currently there are no plans to add VLC to Plop Linux. Maybe, when I have a lot of time, then I will release a build script for people that want use VLC.
Regards
Elmar