Hi there!
1) No you cannot scale an entire design. Though, you can scale up and down a custom component. So if you save your whole ship as a component and then make a design with only that component, you could scale it up or down. Not terribly useful I'm wagering, and some components won't obey this when part of a custom component.
2) Nope, for now, you can't make a race use your own designs by default. At least not without modding.
3) Thrusters add tactical speed, we are not certain exactly how they work, but generally, I find they reduce opponent accuracy, so I often make tiny and small hulls that forego defenses for thrusters, so that they can be useful against all comers. Furthermore, timing matters in battles, so if you have some carriers and you want a few of your other ships to attack WITH your fighters, they will need some extra tactical speed to keep up.
