So, plotwise...
The Engie: Very logical speech, good machinists, generally peaceful. Loyal to the federation, have a pact with the Mantis. Have not done well since the fall of the federation. There are no primarily Engie pirates, though they may join pirate crews.
Mantis: Not friendly to the federation, but also not hostile to them - so long as they stay out of Mantis space. Enjoy hunting Slugs, who they hate, and Rocks, who they see as worthwhile enemies. Very frequently found as pirates.
Zoltan: Independent, strong believers in the letter of the law and quick to deal with potential threats and criminals. They have some advanced technology, and the Zoltan wise men can be alternately destructive or helpful, depending on their mood. Sometimes found as slavers or pirates.
Slugs: Disliked by everyone. Often found as pirates and slavers, and fight through trickery and subterfuge - often risking interspecies incidents by pretending to be a race other than their own. Particularly hateful of the Mantis.
Rock: I know nothing about these guys. Fairly common pirates. Like to fight, but do not use subterfuge like the Mantis and Slugs.
I like thinking about stuff like this, even though it's so up to personal interpretation and first impressions. Like I figured the Engis on Mantis ships were technically slaves, but that they don't particularly mind as long as they can repair things.
Zoltan attack me an awful lot for being diplomats, so I see them as DF elves or otherwise fake pacifists. Pompous and practically bloodthirsty despite showing up as "friendly".
Meanwhile the Rock seem peaceful, if isolationist. I'd take a Rock sector anyday, despite their missiles tearing up my shields-over-engines strategy.
Slugs are stuff of legend, nobody actually LIVES in nebulae and only fools try to navigate them. (I hate not having sensors so getting a slug crewman is ever so nice)
I tried the Rock ship today. Went really well as soon as I ditched the missile launchers for energy weapons and a bomb. Relying on missiles drives me crazy. Fire beam with rock boarders is as fun as everyone says, oh yes.