Remember: MAME 2003-Plus is not MAME 0.78. It is a hybrid. Therefore, you cannot use a pure MAME 0.78 set (too old, missing Plus drivers) or a MAME 0.260 set (too new, files renamed).
There are several advantages to using full non-merged ROMsets in MAME 2003-Plus: mame 2003-plus reference: full non-merged romsets
This is where most users give up. Let’s break the bank of jargon. Remember: MAME 2003-Plus is not MAME 0