I am planning a game were the explorers are ambushed by xenomorphes aka the alien from Alien any comments or advice?

Tyranids? Nuff said.

Or more specifically Genestealers.

Still if you want something unique feel free to make something of your own up as yet undiscovered. The Koronus Bestiary might be able to help too.

I am planning a game were the explorers are ambushed by xenomorphes aka the alien from Alien any comments or advice?

Genestealers are very much a Xenomorph equivalent.

The key questions will be:

1. Ambushed where? On a world (if so, why are they down there) or on their ship (if so, how did they get on board)
2. How many are there? A tidal wave of stabby monsters (aka Aliens ) or one/two supersneaky ninjas (aka Alien )
3. How are the dozens or thousands of armsmen a Rogue Trader might call upon prevented from interfering?
4. How are the explorers supposed to beat them/escape?

Having the explorers 'cut off' is probably the best bet - allow them to take along a few dozen bodyguards (who you can murder in suitably nerve-twanging fashions). In one adventure I recall a salvage rights document requiring the rogue trader to be the first person to set foot on the derelict. All you then have to do is figure out how to cut them off from their ship....

Genestealers are unnecessarily vicious to PCs; I hope yours have a fun time of it

I also REALLY hope that they get aboard the ship. There are so many dark places aboard a voidship to hide, and whole groups of people aboard who no one else might ever see, if nothing goes wrong. They could have little Genestealer Cults crawling within the bowels of the ship for a long time, and then even after the Genestealers you know about are dead, more hid in the oubliettes within the ship, quietly multiplying. Someday, they can make the ship crash on a planet, and spread there.

A Lord-Captain who is secretly a Cultist, or a Humanoid Genestealer, would be terrifying.

I also REALLY hope that they get aboard the ship. There are so many dark places aboard a voidship to hide, and whole groups of people aboard who no one else might ever see, if nothing goes wrong. They could have little Genestealer Cults crawling within the bowels of the ship for a long time, and then even after the Genestealers you know about are dead, more hid in the oubliettes within the ship, quietly multiplying. Someday, they can make the ship crash on a planet, and spread there.

A Lord-Captain who is secretly a Cultist, or a Humanoid Genestealer, would be terrifying.

Have a listen to the Deathwatch Audio Book Mission:Purge , which has a fairly similar plot.

Still if you want something unique feel free to make something of your own up as yet undiscovered.

-And there is the 'onion-skin' option: using the stats of a critter that works like you want it to in the scenario, but describe it's appearance differently. The stats of a Genestealer could be applied to huge spiders, hairy berserkers, tentacled horrors, etc.

Someday, if I can find a good way to cross ship-actions and maybe space combat with something that involves Nids, I wholly want to use the Void Kraken stats to approximate something akin to a Tyranid bio-ship. I would add some guns, the Shadow in the Void stuff, and whatever. A Rogue Trader and Co., or maybe some Space Marines/Terminators could have to board it, and fight their way to something to destroy it. A shame, but a good shame, that Nids aren't in RT; but they might win in Koronus.

Koronus bestiary includes a huge tentacly void-monster, useable in space combat...