>>11837524 (OP)It's not an engine limitations. There are a lot of mods that allow to put several enemy types per room.
The issues lie elsewhere. You have to consider that if you put several enemy types together, you need to work the collision and behaviour between them. Also, in RE1, if you get surrounded by 3 enemies (Zombies, Hunters), you can get bit continuously and get stunlocked to death (doesn't matter how much healing you have on you you're only prolonging the inevitable). By having several enemy types together you're increasing the chance of that or other unfair scenarios occuring.