So sometimes crewmates will have animalistic features. Non of the show crew has these [mostly cause I can't think if any]. These develped likely as just traits from before their species became advanced. Specifically the crewanimals [crewfish for example], evolved to what we have now :] as to why every crewmate has the same body [I'll add body types to my style, i mean just like the general bean shape] is likely just the everything turns into a crab eventually thing but its crewmates /silly but can be the explanation

They don't need to be specific, and can totally be of mythlogy/fantasy [unicorns and dragons] and can also be of a unnamed species [pink my plant creature <3]

They don't have paw pads/hooves [and if they did it'd be inside the suit], but they can have light patches on their hands that resemble them. They can also be on there body. They can also have tails and wings, but no fur [on the suit], and no digitigrade/other types of legs cause trying to draw that gave me headaches.

Imposter traits [mostly when in crewmate form]

includes claws and the mouth. The claws are cause by tendrils poking out there [think poking your finger through a balloon], as they know that's a free weapon. The mouth + tongue is for quick killing, like stack killing or needing to quickly vent. It does damage their skin suit thing but they can usually melt the suit back together. The tongue is just a really sharp needle one, think a porcupine quill but as a tongue

Though most times they just come out of the shell, as it's easier to kill if you overwhelm your victim in your true form, plus you're likely faster

For the rhm kill animation, it's just someone had cybernetics, and the imposter took advantage of that. Now how common cybernetics are, idk, technology seems advanced [if a bit junk], so maybe

In the show they often don't use weapons, and I think that's cause they just didn't have any. In situations where they join the ship as an imposter, they can bring weapons by sneaking them on [like a knife cause they're cafe staff, or a gun cause it's part of the weaponry they were finicking with] plus I doubt mira cares as long as you have a good enough excuse