I used to use the old nimbus's when I went to school, so I'll clear up some mis-information.
They are mostly IBM PC Compatible (the machines at school ran MS-DOS 3.3 and could even run windows 3.0), its the video and some other BIOS pieces that are not. It uses a custom video thats like EGA (but only has a fixed 16 colour palette). There is a pc loader piece (called setpc3) that basically installs a TSR to fix the bios and video routines to make it more compatible and it adds CGA graphics support (whilst it works, there were a few games that I remember it not being able to run)
Because of this custom video it has a custom video BIOS, which includes a few routines to do lines, circles, fonts (all from bios calls), which if you remember the old login screens, or used rm-pascal, rm-basic, logo etc., they used the graphics routines for that.
Later when RM produced 486 machines, they didn't include the custom video (they used standard VGA and SVGA graphics cards), so they made a program to provide the old BIOS routines to be able to run the RM software. Its called RUNPC186 and the original version checks something in the 486's BIOS to make sure its an RM machine, there is a hacked version which removes the check so it will run on any machine. Sadly though it eats a fair bit of memory, but I've yet to see that being a problem.
Sadly it doesn't appear to work on dosbox, but using vmware it works fine. The com files that you are talking about will actually work fine under dos (they are old adventure games, those extra files are data for the game), because they appear to not use any special RM routines. If you try to load BBCBasic it wont work under dos, but it will under runpc186.
Hope that helps.
PS. they do recognise dos formated disks, but only 720k variety, they didn't have a high density disk drive, so they couldn't read 1.44mb disks.