It's a nice idea, but for it to be safe, it needs to be more "on rails" than you'd probably envision.
Think about it - If you're in a confined space with an Alien or Predator, it's going to be damn hard to hit your target. It's either going to be almost invisible, or clambering around the ceiling and walls faster than you canfollow.
If, on the other hand, you're in a confined space with a guy in an Alien or Predator suit - He can move no faster than you... Slower, in fact, due to the cumbersome costume and limited visibility.
So you need to have a nice big "confined space" with plenty of cover for the creature to hide behind for starters.
You need to be able to control where the group go - Unless you have a large number of creature performers to ensure gameplay in all routes through your game area.
Even then, you need to have the game laid out in such a way that the creatures never get stuck between two groups of humans.
Basically, even if you iron out all of the H&S problems, you also need to try to break your game.
You need to think through your scenario from every possible perspective, and close every loophole that could force a premature end to the game.
Then look for more loopholes.
I've been running an Airsoft site for about 8 years now, and I still often end up with some smartass player coming up with some way to circumvent everything we want them to achieve in a given scenario.
From the scenario you mentioned above - The alien appears, and is shot.
He retreats...
The Marines move forward - Now knowing where the Alien is likely to appear from.
He appears again, and is shot again.
It's not gonna work.
As an example of a similar scenario:
When we do Zombie Games at S8, we only do them after dark.
This is to give the Zombies some chance to beat the human players.
Otherwise armed vs unarmed = no contest.
The Zombies must take one step backwards and pause for a second when shot for the first time.
When they're hit a second time - Two steps, and two seconds.
And so on.
After ten seconds, they have the option of retreating to a Spawn Point and starting over - Or they can keep getting body parts blown off and taking longer to regenerate for an attack. Up to them

The point being, the Zombies CAN attack the humans effectively by using cover of darkness, but the humans have an equal chance of defending themselves if they stay calm and use their ears.
Any game with armed players vs unarmed players must either be balanced in such a manner, or be set up as a simple target shoot.