Well, the idea isn't for the machines to do all of the fighting.
They will be activated to join the humans in the safe/locked down section of the compound and do some reconnaissance on their way over.
The storage bay where they are being held is on the other side of the facility.
Now in terms of the questions:
How do the aliens attack?
The aliens can most easily be likened to the Xenomorphs in Alien, though without the part where their blood doubles as an acid.
Their appearance will be more like a bipedal crustacean, around ten feet in height.
Their attacks will focus on their superior strength and the sharpness of their claws (as well as sharp edges on their exo-skeleton).
Said exo-skeleton will also work as alien kevlar against low-caliber gunshots. It will take a lot of high caliber rounds or a small explosion to bring even one down.
When they attack a replicant, will it be killed?
If not, how do the replicants survive?
Replicants can die if they sustain too much damage. Since the aliens are strong enough to dismember and literally rip humans and replicants to shreds, that means that replicants can and will perish.
Obviously though they can survive wounds that would kill a human almost instantly. They are no longer salvagable when their memory core (which is located in their head) is completely destroyed.
So even a beheaded replicant can still have his consciousness loaded into another body, but once the memory core itself is destroyed, the replicant is essentially dead.
Is it possible for a player to have two characers, one human and one replecant?
Whether or not I allow multiple characters depends a bit upon how many people are interested in this game. Right now I am leaning towards "yes".
Is it possible to play a non-human who is allied to the humans?
The non-humans have no interest to ally themselves with the humans. In essence, their intelligence is primitive (though they do have a form of communication that allows them to work in units).
They perceive the humans as a treat to their territory and their only response is to wipe them out.
How will the replicants go about investigating?
The replicants will search the sections of the base they need to pass through to reach the trapped humans. Some portions will have limited to no power, others will still be fully functioning.
Likewise, not every section will (already) have an alien presence.
what will the humans be doing?
The humans will activate the replicants remotely and then initially wait for the replicants to reach them. Once they have, they will use the date the replicants have gathered and devise a common strategy to escape.
What forum where you thinking of putting the game in?
I was leaning towards the NC exotic section.
Hope this answers some of your questions! :)