I think what your experiencing is... absolutely normal.
I learned to drive manual in a 2.2L Toyota pickup. Letting out the clutch with insufficient throttle from a start would cause the driveline to buck badly as the ECU fed the engine fuel in an attempt to keep it running at below-idle RPMs.
Unless you're saying that 1st gear is engaged at a higher engine speed than idle (like, during gear braking), and applying throttle causes the car to buck. In that case, I have no idea what your problem is. But if you're bucking while applying throttle at between 5-600 RPMs, I wouldn't lose any sleep over it.