I got this achievement yesterday, and the difficulty is that it is relatively random. However, I'll share my anecdotal advice.
First, hit all the shops you can. Shops don't always have crew, but there's a chance that they might. I've found that shops in a sector belonging to a particular race tend to have that race more often, but it's random - sometimes I've been in Engi space and seen a shop full of Zoltans, for instance. Crewmembers tend to cost less than 100 scrap, but I'd suggest trying to have ~100-150 scrap on hand just in case you find a shop that's got 2 races you need.
One event that I believe is more common in "pirate" and "civilian" sectors is an encounter with a slaver. They may attempt to extort you to give them either some scrap or one of your crew members. If you fight instead, they may later attempt to surrender by offering you a crew member.
From time to time when fighting a ship and killing its crew, I've been told it was a prisoner transport ship and that one of the prisoners wanted to join me. I'd suggest attempting to take ships intact for this reason. I haven't seen any indications in the initial system event that would indicate whether a ship is likely to give a crewmember if taken intact, though.
There are also events where you find a lone survivor on a deserted station, and this can sometimes provide you with an additional crewmember. However, there's also a chance that they will go crazy and murder one of your existing crew, so this is not always a reliable way to make progress towards this achievement. Commenters have noted that having a level 2 medbay can counter this, as can a slug crewmember.
Different areas feature different events, encounters and nodes.
- Nebulas will have approx 3+ nodes that are NOT sensor-disrupting nebulas.
- Homeworlds feature specific events that allow you to unlock some ships.
- Pirate sector got plenty of pirates (totally unpredictable)
- Race-controlled sectors will mostly have that race pilots (good luck boarding a Mantis crew of 5)
Although it's randomized, it doesn't take a load of thought to know what you'll find. Zoltan sector = loads of Zoltan = easy to teleport in and slay em. Rock sector = loads of stoners = teleport in, kite them around the ship, breaking their systems (they're too slow to ketchup)
TL;DR: Yes, there is a differense between regions, and red regions have more hostile encounters than green ones.
Best Answer
That all depends on how beefy you are at the point that you choose where you're going. Civilian sector doesn't equal completely safe. A hostile sector, while a bit more unsafe, also generates a bit more reward for being risky. This is in the form of special encounters and shops. Also, as said before, if your ship is decked out and has more shields then most things do, you might be safe enough to trounce anything in a hostile sector, thereby gaining valuable scrap for nothing. Also note that you WILL have to defeat a large opponent at the end of the game, and you will NEED to have your crew leveled up on Helm, Shields, Weapons, and Engines before then. If you avoid every battle, this will never happen. Getting into fights and having them dodge things or charge the weapons is the only way they get exp. If you're having a hard time on a particular run of the game, choose neutral sectors. Otherwise, I would go for the more hostile ones myself.