You can delete any game you don't want without breaking others. Moving single games is incredibly easy.
If you download a MAME 0.240 ROM set and notice that games like Killer Instinct , Area 51 , or Blitz 2000 will not load, it is likely because you are missing . mame roms set 0240
A complete MAME 0.240 collection consists of both the (a few hundred gigabytes of small ZIP files) and the CHD set (several terabytes of larger disk images). CHDs must be placed in specifically named subfolders within your main ROMs directory to load correctly. How to Verify and Manage Your 0.240 Set You can delete any game you don't want
One of the most common frustrations for new MAME users occurs when launching a game only to see a black screen for a second before being returned to the game selection screen. This often indicates that the game you're trying to run is a clone, and the parent ROM is missing from your set. A complete MAME 0
If you are building an arcade machine, LaunchBox is going to make the process much simpler. It will use whichever ROM set you managed to acquire and automatically match it as it installs MAME for you.