That was two generations ago (The last survivors of the Nanopocolypse are just now dieing out as their grandchildren reach adulthood). Also, alot of colonists died after the Nanopocolypse, when anti-nanobot sentiments spread, alot of colonies instituted a blanket ban on nanotech treatements. This meant that all the nanobot-based medicine the Nakeesh had been relying on was no longer used, and lots of people died as researchers raced to find non-nanobot based cures for illnesses nobody had really even noticed before. Not to mention that they were no longer using Nanobots to purify their drinking water, treat waste, make air breathable (Okay, they didn't shut the air scrubbers down immediately, but in alot of cases they rushed the job) grow food, or do lots of other things they counted on. Some colonies were totally wiped out in one way or another.This isn't really consistent with what you've said previously. You said that there were 6 billion Nakeesh in colonies, and 20% of the 10 billion on Nak escaped (so, 2 billion, which fits your 'current population' total of 8 billion). One would expect that the 6 billion already set up on colonies would remain in their homes, and so we only have 1/4 of the population in fleets, assuming that none of those fleets joined or formed planetary colonies. Not really a species of space nomads anymore.
And if "most of" the 8 billion Nakeesh (which, incidentally, is still larger than the current population of Earth by roughly a 6th) are in communities of less than 10 000, we're looking at a minimum of 400 000 communities for that half of the population. That would mean there's 100 000+ space fleets.
Also, the destruction of Nak caused alot of unrest in the colonies.
There were quite a few bloody civil wars. The Nakeesh colonies had always been closely tied with their homeworld. With it gone, alot of wars started, usually between people who wanted the colonies to become independent and those who wanted the colonies to remain governed by what was left of the Nakeesh government. Not to mention wars between groups with varying opinions concerning nanotech. Many colonies were totally reliant on trade with Nak, and were abandoned or sold with the planets destruction. Then, once these bloody civil wars were settled, these colonies got flooded with starving refugees, which in many cases caused even more problems.
So yeah, that 8 billion isn't Nak Survivors+Colonists. It's the children and grandchildren of those colonists and survivors after years of conflict, struggle, and chaos.
And 8 billion is more than earth's current population, the population on one planet. Spread out amongst the stars that's not much. All the other races probably have alot more than 8 billion disorganized people.
Though if it makes you feel better I'll decrease the number of surviving Nakeesh/ those in the colonies or increase the number of fleets.
Edit: Okay, how do these numbers sound. 8 Billion Nakeesh alive after the destruction of Nak. Since then Conflict, disease, starvation, and the population simply not being replaced fast enough (You'd be surprised how bad "Hey baby, our homeworld is destroyed, our civilization is ripping itself to shreds, and the technology that we built our greatness on is now taboo, and everybody is sick, hungry, terrified, or trying to kill one another, lets make babies" works as a pick up line. Maybe it doesn't translate well into whatever language the Nakeesh use). Anyway, current population total is at around 5 billion.