I haven't heard of the HMG7900, but i didn't take much notice of the avalanche of new computers at the time. My interest then was hacking CB radios.
I've never possessed a coleco cart, so couldn't really say, but i think the slot is supposed to be the same size.
I've got 6 demo tapes that came with the computer, but i haven't been able to get cassette loading to work as yet.
There should also be a SD-BASIC 1.0 cart around somewhere in the world.
The Basic cart I have, has 2x 2764 ROMS and a 2732 ROM. No tricks, nothing special. I dumped everything by connecting the printer port to my Super-80 and writing a program to transfer everything over. From the Super-80 I then saved it to a 386 pc, then to a floppy disk, then to this pc for uploading.