Just because a species looks a certain way doesn't mean their ships looks that way. The ishiman's aren't blue; the Aud ships and auds are the same colour, but that's because they're both machines. Just because the grolk and gaitori have similar looking ships doesn't mean they look similar. My impression of the Grolk is that they are probably somewhat large, a very physically developed race. I see their heads being somewhat small, with large eyes and ears. My reasoning for this is that their ships are bad. Because they obviously developed in isolation (not being in the solo missions leads me to believe that they are isolated in the galaxy), I think that they were able to get by on their homeworld almost exclusively by their physical evolution, and gained sentience as a sort of evolutionary fluke. Their ships are artless, utilitarian looking things, so they probably aren't all that culturally advance. I'd place them laterally on the evolutionary scale to humans, perhaps slightly lower, looking like something evolved from a mix of dog and lizard.
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We woke up one morning and fell a little further down, for sure this is the valley of death.
I open up my wallet, and it is full of blood.
-Godspeed you Black Emporer!, Dead flag blues