But I wanna ask, do people make rom hacks for DOS games, and other old computer games like Commodore 64, Amiga, Tandy, etc.? Is it possible for people to hack old DOS games, the same way they hack NES or SMS games?
Many things you will see there are mods for things with level editors and also things which have since had source code released or remade.
Still "understand file format and programming used by game, edit said file formats and programming" works just as well for X86 as it does for MOS, 68K, ARM, MIPS or Z80. Debugging might be slightly different as you tend not to run things within a fully self contained emulator, though with some tweaks on dosbox and if you prefer a virtual machine you can do that if you really want. That is OK though as slightly older X86 has probably the best and most comprehensive options for debugging of any platform, IDA is the thing most geared towards it but you have many many other alternatives. Tile editing is not really a thing as PC graphics are not really a fixed target like they are for consoles ( https://www.youtube.com/watch?v=niKblgZupOc
) and high level formats were not unpopular, you still have the concepts of tiles, textures, text, audio, 2d graphics, 3d graphics* and whatnot to consider.http://wiki.xentax.com/index.php/Game_File_Format_Central
also is a thing and much of it is concerned with PC games.
*if the dos era ended with windows 3.11 or you want to go into the 9X era then there are a variety of 3d formats that are not opengl or directx/direct3d. For my money the big two are 3dfx and s3 virge.
Unlike consoles and this place then many things will instead happen as a genre (things like visual novels or doom clones) or individual game effort (I mentioned source remakes already but individual games themselves have things) or indeed older level editors being expanded to cater to more modern sensibilities.
So yeah if you know what we do/teach/practice here you can probably jump into DOS era stuff well enough, PCs of the time were not necessarily more powerful than consoles or arcade ( https://www.youtube.com/watch?v=7YreEwtV7D0
) and again you have level formats and operating systems on top of things to consider, which is probably where your barrier to entry will be.