The source of the alien negative mental effects

Use M24s at max range. The Centipedes will NOT follow you IF you manage to stay out of their range. Focus the scythes first. They have the range of the bolt action rifle, but you can kill them easy with the 3 tiles more range of your M24s. Later focus on the Centipedes. They are very much no problem but start with Inferno > Mini > Blaster. Against 20-30 Centis and same amount of Scythes you can manage to win without receiving any hit at all. But you need half a dozen of snipers and a few in-game days to clear them.

The tactic does not work with attacking Mechanoids. I have 40 Centis attacking my base and no idea how to fight them down. Any suggestions?

You're off to a good start next I'd get some defenses up, turrets and sandbags to be more specific. This will help with raider waves.

Which brings us to our next part: getting more people.

You can get people in 5 ways right now.

1. People will wander in and just join up no questions asked
2. An escape pod will crash and you will have to rescue the person and convince them to join
3. When fighting raiders, randomly the killing hit won't kill them only knock them unconscious. When this happens you have the ability to capture them
4. The same can be done for visitors
5. The same can be done for the people in cryo-sleep pods. A warning about this one though those pods are guarded by some hard to kill and hard hitting enemies open at your own risk

When you capture anyone you need to have prisoners beds set up for them to sleep in. I usually set these up away from my main colony just in case one breaks out. You then need to have a warden "try to recruit them" your chances are better if the person is not injured and well fed.

As from where to go from there you could outfit people with bionic parts, set up different productions to earn lots of sliver or build a rocket and escape the planet.