Marines: Fish bowl space suit dudes with rocket packs and big rayguns.
IG: Red shirts with funny helmets (maybe with a wing on top), goggles and small rayguns. 50's military gear painted in silvery colours and fitted with blinking lights and antennas, because they're 50's in SPAAAACE! Just replace machine guns with rayguns, cannons with big rayguns, etc.
Sisters: Your post war Rosy the Riveter gals with spunk, but still like to be called pretty and slapped in the ass from time to time. Can hold her own, unless there's any significant male character around, in which case they're shit.
Inquisition: Feds. Show up, flash a badge, make you spill your beans or tell you there are no such thing as flying saucers.
Admech: Science! Weird Science! Mad Science!
Navy: Bunch of pipe smoking, "Son, dem ruskies will not wanna talk to ya, so better git shootin'," types.
Eldar: Old and wise race that sees humanity leading up to their doom, because that's what they almost did. Plan 9 from Outer Space, The Day The Earth Stood Still, etc. Look human, but dress in fancy space alien suits. Their society has grown out of their primitive impulses (no they haven't) and take pity on the warring factions of the galaxy, like they were children.
Dark Eldar: Ming the merciless arch type?
Tau: Yes, bug eyes martians with fancy rayguns and saucers. For Science! And all the auxiliaries? Enslaved aliens or mutants made, through science!
Tyranids: Big space bugs. Attack of the Giant Spider, Blob, etc. A hive fleet is suppose to be a singular organism, so a big Godzilla is as good as a bunch of little space ants.
Orks: Any monster that's suppose to be smart enough for space travel, but acts like an animal when humans come along. What was that black and white movie that Carpenter turned into The Thing?
Necrons: Robots are good. Something like Cylons, but with more of a 50's look.