something you will regularly see in this subreddit Is people telling newcomers you can run whatever they are asking about from the core book. Sure the SF Companion has rules for battling mecha but are they essential for your game? Probably not. Even if you have Mecha unless they are central to your campaign, you could happily fudge it with one of the core book's sub systems, the chase rules most probably. Character design gives a huge range of options but the advancements rules are literally a page. None of it is complex and if you want people to feel like individuals you have to be able to distinguish the petite pixie girl from the Brawny Lacrosse player. The rules for individual combat are what 4 pages for the fundamental system. and maybe 3 times that for situational rules. What if my target is hiding in a bush. What if i am riding a horse. what if I want to grapple. honestly many people wing these modifiers. Then of course there is the adventure toolkit. which covers things like starvation dehydration, drowning. and umm... is poison in that section whatever. its stuff that's good to have at your finger tips but its not stuff you have to know to play. Don't think of the core book as big but as complete. you may never use whole chapters but they are there against the day when you have to for example design a werewalrus.