Let's take these one at a time.
Possessive form of words ending with sibilants (s, ch, sh, z or x)
Possessives of a singular noun are always formed by added an apostrophe and s, even if the word ends in a sibilant sound.
Jack found Anubis's frumpy hood-and-cloak ensemble to be more laughable than threatening.
The Nox's homeworld was deceptively low-tech in appearance.
The SGC was hoping the harcesis's knowledge could bring down the goa'uld.
The imprisoned team was mightily relieved to hear Col. Reynolds's voice on their radios.
Please note that it is never "Col. Reynold's voice"—the s is an integral part of his name. [Hint: When in doubt, switch formats to "the voice of Col. Reynolds." However you see the name/noun written, add the possessive after that.]
Most style books do permit the dropping of the s if it makes pronunciation extremely difficult or awkward, especially classical or biblical names ending in es, is or us. Jesus' and Moses' are commonly allowed exceptions (though I and many others personally say and write Jesus's). Other noted exceptions: Achilles' heel, Hercules' strength, Ramses' reign, Xerxes' conquest, Isis' temple. With classical names such as these (which can often come up in Stargate writing!), try speaking each option out loud and use your preferred form consistently throughout your fic: Does it sound like "Osiris ribbon device"? Then write Osiris' ribbon device. "Osirises ribbon device"? Then write Osiris's ribbon device. Use whichever one sounds right to you.
Possessive form of plural words ending with sibilants
Plurals ending in an s take only the apostrophe to form the possessive. (Do remember to pluralize your sibilant-ending names first, though. Col. Reynolds and his family are the Reynoldses.)
Daniel peered over his glasses' rims, looking bemused.
The Carters' trip to San Diego was cut short by a Tok'ra emergency call.
Watching the Lees and Coombses at the Meyerses' Festivus party made Felger all too aware he was the last single man in the science department.
[Hint: Again, try the format switch: the party at the Meyerses. The apostrophe goes after the plural form indicating the Meyers family.]
The Chicago Manual of Style. 2003.
The American Heritage Book of English Usage. 1996.
The Elements of Style. 1918.