You can make a really big vehicle size, and a really big planet buster comp that only fits on that size. This could simulate needed 9 ships or whatever to break planets, but it would introduce other problems, like using the big vehicle size for other purposes. Of course, you could give it 1000 increased maintenance, and give the planet buster comp 1000 decreased maintenance to balance that out. That would strongly discourage people from using the large ship for anything else.
