Planets in the background are harmless, they do nothing beyond looking pretty.
Mules are good as meat shields with modest attack power early in the game until you progress long enough to build your first colony. They will not be of use for much longer afterwards. Until then, a wall of Shrike (P), Enforcers, and Mules recovered from pirates will get the early-game combat job done.
If you really want an armed freighter style ship that is good for most of the game, then Apogee is the best ship for that job, and it is good enough to be useful for the entire game. Even better, you can start the game as a Apogee, although it will be underequipped until you find better weapons. It will not be an offensive powerhouse late in the game, but it is a great shield tank and it can survey and haul on the side.
You get a colony alert telling you how many days you have before they arrive.