OK, many thanks for that information! I'll try very carefully to install P96 and RTG ASAP.
Meanwhile, I've been working on getting some other Cores working. I've now managed to get the Sinclair QL and C64 Cores working. I think in both cases this was due to installing different versions of their Cores, but I may also have renamed the QL ROM. The C64 Core is another one which seems not to do anything until you press F12. I think I'll try these techniques with the Tandy "Coco" and the Oric as well. I still don't know how to use .mra files, though.
I also found out I had no mist.ini file on my SD card for the non Amiga Cores, so I created one to try and tell some Cores which ROM files to load, but this had no effect! Unfortunately, using my MacBook with the software Pages and TextEdit, I was unable to stop the extension .txt from being appended to mist.ini. I renamed it several times, but the .txt kept reappearing, so this may have prevented it from working. I'm worried that some Cores may not be detecting my 64Mb RAM and defaulting to 32Mb, so that might be why they don't run. I also wonder if I can upgrade to 128Mb RAM.
I don't think I've found the source code for any Cores yet. It looks like various people have already created Cores for all computers that have ever existed, including some eastern bloc computers I'd never heard of before (e.g. TVC Computer). Unfortunately, I think some of the Cores available don't run on the MIST, only on the MISTer, but I'm not sure which ones these are.
