I wouldn't put a hatchetfish in with a pearl gourami, from experience.
I have 6 kuhlis in my 20 gallon, they go nuts with dimmer light, swim all over. They will bump into any fish that doesn't move, but they will never nip them.
Hide a lot when the tank lights on though. Just set up your tank so that the hiding spots are still visible to you.
With the betta in with tetras; It's rare for the betta to be aggressive towards them, usually the other way around, and the betta gets nipped. Which may make the betta aggressive towards them.
If you do look into small cichlids, I recommend Kribs. Get a male and a couple females and your set, if you have caves, you're set for breeding.
If you can't fit more than 2 kribs into your stock, just wait a couple weeks until the male picks a partner, and remove the other females. They will be a pair for a long time, meaning lots of fry, and possible money!