Provided two figures have the same TA, If one has the Force Field feat card, can that fig be used with the other (having super strength) for the Fastball Special feat?

No. Figures must meet the prerequisits for feats at the time of force construction. Even though Force Field grants Toughness, the figure with Force Field doesn't actually have Toughness until the game begins. Since there isn't any Toughness on the dial during force building, the Force Field figure can't be attached to Fastball Special.