The upcoming Blade Runner game has a really interesting approach to damage and critical hit. It is closely related to T2K, and perhaps this system could be adopted to Alien too. For those of you that haven't read the new BR game, then critical hits are triggered by a good shot/well placed hit, you get an excess number of success on your hit. The critical hit table is listed from 1 to 12, the dice type (d6, d8, d10 or d12) you roll on the critical hit table is based on weapon used. In BR a subcompact pistol uses as d8, a magnum revolver a d10, the blade runner blaster d12.
The crit tables have instant fatal results on a result of 8, 10 and 12, so a d8 have a 1 in 8 chance of being instantly fatal, a d10 a 1/5 and a d12 a 1/4.
So what is needed to to create a crit table for xenos (I wouldn't use the same as for humans, but you could) and then set the crit dice type for the diffranet weapons, a VP70 probably a d8, Shotgun and magnum revolver ... d10? and M41A firing amour piercing explosive runds? defiantly a d12.
The explosive/blast system in BR is the same as in T2K and I think that is much better than the system in Alien, but that is another story.