CHS is event based and fires after game start, so your initial look will not indicate colonizable worlds until you play the game. The first event gives everything in your home system 10% habitability for you, and fires during the first month. The second event fires at the first month turnover and gives an additional 40% to allow you to colonize, but that modifier disappears once you actually colonize the planet. So your first pop will get 10 percent happiness from the 10 percent home system habitability modifier.

Build a hab building, and the planets hab won't change, but your pop will go to 25 percent happiness due to habitability. Build more hab buildings as needed until they settle down.

You'll only get 25 tiles on oversized (26+) worlds, but they do exist. You can garrison them with oversized numbers of armies. Unclear whether now with global food you'll get invisible unemployed pops past 25.