Interesting project, but its going to be extremely difficult. A good reference may be Grevious1's Whispers In The Dark (search it in the maps section). Its based off the game (to my knowledge) and has some features that would make your life much easier in replicating Dead Space.
As for the actual idea: 4 necromorphs? I might actually have to say that you should follow the versus zombie maps way, and add normal and player units. 4 necromorphs on a decent sized ship will end up with you being bored or overly paranoid (omg... no attacks in 5 minutes! WHATS GOING TO HAPPEN!!!). It doesn't have to be much, but a wandering CPU necromorphs here and there is going to stop you from being bored, especially if the other team is afk.
As for the no ammo aspect, how are you designing weapons? Like the "mining equipment" from the original game? That would require some sort of power input, which will lead you back to the "ammo everywhere" feel (the fact that in the game, every employee had some kind of ammunition in their locker, and every person aboard the ship had ammo or money in their massive luggage boxes, with nothing else). Or you going to have it like literal mining equipment (not projectile based stuff)?
Lastly: Some tips for the atmosphere: heavy fog and lighting base. The terrain/doodads are second to those. A hall is much scarier when there is little light, and heavy fog [alone, no doodads], then if the same hall had constant dim lighting, no fog, but blood everywhere on the wall, and dead bodies everywhere.
I'll be watching this with hopes for success

PS: "They use the vents *static*"
