NO, A Fiero will NOT combust if hit from the rear. The gas tank is in the middle of the car. In order for the gas tank to rupture the entire car would need to be crushed or cut in half. At that point bursting into flames would be the least of your worries.
A Fiero has a 5-star crash rating for front and rear and a 4-star rating for side impact.
As for a "good" car, All Fiero's are over 20 years old. How well it was taken care of over those past years will determine if it's a "good" car.
As for leg room, The Fiero has more legroom than 80's/90's Corvettes.
I know folks as tall as 6'8" who have them. But it should be noted that the factory sunroof cars have almost 2" more headroom that the non-sunroof cars.