A simple mod, due to it using editor numbers instead of replacing actors directly it should work with most if not all mods for Doom and thus adding their Doom 2 exclusives to Doom 1 too! for balance reasons, the Doom 2 monsters have a lower chance of spawning in place of the Doom 1 monsters. so maybe you won't see all of them that quickly.
Known issue: if a mod ALSO replaces the editor numbers of the Doom 1 monsters it will NOT work with this, also make sure to load this BEFORE any mod you wanna play.
what spawns where:
"chaingunners" spawn in place of the shotgunners
"revenants" spawn in place of the imps
"hellknights" spawn in place of the pinkies
"arch-viles" spawn in place of the spectres (rare tho)
"pain elementals" "Arachnotrons" and "Mancubi" spawn in place of the cacos
"supershotgun" spawns in place of the shotgun (somewhat common)
"megasphere" spawns in place of the soulsphere (very rare)
Screenshots (old):


more at: https://imgur.com/a/iWbS5zp
Link:
https://www.dropbox.com/s/pa8nao3v02y35 ... 2.pk3?dl=0
Quick Changelog: tried to rebalance the monster spawns a bit more, added ambush flag compatibility, and finally added in Doom 2 sprites and sounds since I don't think it should bring any legal issue (other mods also have these sprites and sounds despite working with Doom 1 so)
Older version:
https://www.dropbox.com/s/jvcncxwgtt9y4 ... 1.pk3?dl=0
https://www.dropbox.com/s/8kp9ji4ym2e10 ... 1.pk3?dl=0
credits to Id Software for the Doom 2 monsters
Freedoom phase 2 monsters for phase 1 taken from: https://freedoom.github.io/index.html
Special thanks to Skelegant for helping me with the Cacodemon spawner and to Yholl for helping me with the ambush flag issue!