uhm... the VF-0 was going to get the fusion engines as well, but they were delayed... the UN knew about the big bad aliens when the Vf-0 was designed since they built it with OT from ASS-1...
and no matter what the fuel, anything that flies in the air and is supposed to be able to carry on the mission of a fighter, is going to be streamlined... so it can go fast and turn and stuff.
the VF-0 is the updated VF-1 design... I'm pretty sure mr K. has already said his original design for the VF-0 was something more old school and in keeping with the era, but decided to go with the current design instead, which borrows heavily from his more current designs and incorporates design ideas from modern day fighter aircraft.