@NinjaKnight When you select a ROM in RetroArch, you should see the screen below where "Run" is the top option. Below that you'll see "Set Core Association". After you select that, it'll take you to all the cores you have installed (which you can do via RetroArch itself) which might open that particular ROM. At this point I'll set it to the current MAME core and if that one doesn't work, I'll try all the others until one does work. Once you assign a core, RetroArch remembers and from that point on all your ROMs will run with their associated cores. In the case that none of the cores work, I'll try to find another version of the ROM and go through the whole process again. It's a slog, but that's the only way I've been able to compile my dream list of games.
Now I'm going down the road of trying to get them all to run via BigBox for an Arcade build.
Note: I have one ROM that I could not get to run at all on any cores in RetroArch (Starblade), so I have to run that orphan from a separate install of MAME 0185.