Let’s solve the "down" problem. You don't need to abandon the 400 slots; you just need to modernize them.
You cannot manually type 400 lines easily. Use a MUGEN SFF/Select Manager Tool (like MUGEN Character Manager). KOF Theory Mercenary Screenpack 400 Slots Down...
Symptoms: A text error saying "Can't load system.def" or "Invalid select screen format."
The Cause: Many links titled "KOF Theory Mercenary Screenpack 400 slots down.zip" are incomplete. They often lack the required data/mugen1 folder structure or have corrupted the select.def grid. Let’s solve the "down" problem
Since I cannot provide a direct URL, here is the most reliable way to find a safe, working download: Symptoms: A text error saying "Can't load system
If your screenpack has custom fonts, frames, or HP bars, don’t change the .air or .sff unless you understand sprite indexing.
Add portraits (usually 25×25 or 30×30) for each char in the screenpack’s *.sff using Fighter Factory.
Symptoms: MUGEN loads to 100% on the loading bar, then instantly closes without an error message. The Cause: The original screenpack was designed for MUGEN 1.0 (32-bit memory limit ~2GB). When you dump 400 high-definition characters with massive portrait files (.pcx or .sff), you exceed the memory stack.