This is the question...in question.
Arqade is an exchange about video games and the question certainly is about one, just on the hardware end.
How does my question about my arcade machine not fit in Arqade?
That's a very good question, because as far as I can tell your question is a perfect fit for Arqade. As the On Topic page of our help center clearly outlines questions about "Game-specific hardware and utilities" are on-topic (emphasis mine):
What topics can I ask about here?
Arqade is for passionate videogamers from all platforms, including consoles, computers, handhelds, and mobile devices. If your question generally covers things such as …
- Gameplay strategies and tactics
- Puzzle solving or obstacle clearing
- Game mechanics and terminology
- Plot and characters in games
- Game-specific hardware and utilities
JAMME certainly falls under "Game-specific hardware" since it's an Arcade board which, as far as I can tell, is used exclusively for Arcade Cabinets. From jammaboards.com:
The JAMMA standard was invented in 1985; any game older than this will not be JAMMA. JAMMA (Japan Arcade Machine Manufacturers' Association) is a standard 56-way connector used on many arcade boards to simplify conversion of cabinets from one game to another. The majority of newer games use a subset of this pinout. Some games (i.e., Street Fighter) which need extra buttons have extra connectors for these additional controls. The JAMMA connector has a .156" pin spacing edge connector (male on the game board).
Honestly, if this doesn't count as Gaming Specific harware, then I'm honestly not sure what does. Sure, Electronics.SE could cover this too, but that doesn't suddenly make it off-topic here.
The only problem with your question was the part asking for a list of 6-player Arcade games, but that wasn't why your question was closed, and I've already edited it out. So as far as I'm concerned your question should be re-opened.
I feel that modifying arcade cabinets is outside of our level of expertise. Our focus is on game playing, mostly, with a little bit of dabbling in consoles and associated hardware.
The hardware in use for arcade cabinets is going to be rather custom work; each game is going to use different bits and pieces, and I suspect even the same game will have different hardware between different revisions. While help with the game itself is something we can easily help with, once you open the cabinet, we're outside our depth. We don't help with modifying PS3s, XBoxs, Wiis, or any other console; we can barely help fix them, and generally answer with, "Take them to a service center." I'm not sure why cabinets should be different.
Knowing which boards are compatible with a specific set of hardware isn't a gaming skill, not in any sense; it requires a level of knowledge in electronic engineering well outside our controller plug and play methodology. For example, if a third party controller didn't work, there's nothing we could do to diagnose or fix this issue; that's outside what we can expect a gamer to know. Sure, there might be one or two of us that actually could answer that, but that's incidental knowledge, at best.