you are right, i don't blame you. the films give very little information on alien biology, so us fans can believe whatever we want. personally i agree with what i think is the greatest alien biology report ever written: The Anchorpoint Essays. Even if you choose not to believe what it says, it is worth checking out. There are logical explanations for every question about the xenos. check it out: anchorpointessays.com
again, believe what you want, it is the designers' faults anyway, though myself and others like daveberg agree with the Essays.