you dont have to import a full romset. install the version of mame you want in the emulators folder (i run my roms from the mame roms folder within this) configure yor mame location in the emulators setup
(mine is running 258) and just drag and drop the roms into the launchbox window - import roms - arcade - use the files in their current location.
I only have 40 Arcade roms on my system.
Hope this helps