fiction - Writing diversity

I am writing a military sci-fi novel about an international military force facing aliens. My cast is very diverse: the MC is Yemenite-Israeli, his love interest is German, his room-mates are from Georgia (the country - not the American state), Mexico and Ireland, the squad leader is Asian-American, and so on. I'm having a lot of fun writing it - the cultural differences are a wellspring of quirks, minor conflicts, unexpected common ground, etc.

However, I feel myself constantly under pressure to make it even more diverse: there's a little voice in my head that goes "there's no black character yet, there's no Muslim character yet" etc. I'm feeling it begins to affect my storytelling: I start to look for ways to insert "member of group such-and-such" instead of looking for what kind of character would advance the plot.

So, my question is two-fold: first, is it a problem is some group such-and-such remains unrepresented? And second, if I have an unpleasant character, and he's the only representative of group such-and-such, would that appear racist?

Answer

Diversity is not, and should not be, a box-ticking exercise. If you're inserting minority characters just for the sake of having them there, you're doing it wrong.

To address your specific questions:

Is it a problem is some group such-and-such remains unrepresented?

Nope.

I am a firm believer that bad representation is worse than no representation at all. As a bisexual, not once have I come away from a work of fiction thinking "That was good and all, but I'm offended that there were no bisexuals in it". I have come away from works feeling offended that the only bi character was a Depraved Bisexual (looking at you, RWBY Volume 5).

I can't say nobody will get offended if you don't include a specific group in your story. But a lot more people will get offended if you include someone of that group and they turn out horribly stereotyped, or are just there as window decoration and don't actually contribute anything. That last point is what I mean by "inserting minority characters just for the sake of having them there" - they need to actually be relevant to the story, and not just "token X". (See also: the Bechdel test.)

If I have an unpleasant character, and he's the only representative of group such-and-such, would that appear racist?

Only if you do it wrong. You need to make it explicit that their unpleasantness is totally unrelated to whatever minority group they happen to be a part of, and again, you need to avoid any stereotypes that would make it look like you're demonising that particular group. Again, if your villain just happens to be a Muslim, and his motive isn't connected to his religion, you're going to offend a lot less people than if they're a Muslim terrorist who wants to wage holy war on the Western infidels.