Wow... where to begin...
Yes Alexander reached India, but he did not stay nor did his presece of that of his successors last very long in India...
Alexander did not create his empire by himself... he had not only the best trained army in the world, (due to his father not Alexander), and he had excellent generals under him who could be argued were the reason for Alexander's success...
...also, Alexander's empire practically crumbled upon his death, while the Roman empire lasted hundreds of years later and left a greater legacy to the world than his short lived one.
You just combined a dozen different scenario's concerning Parthia... Caesar had delevoped plans for the East as did Marc Antony but none of them came to fruition because of the civil wars that occured so that cannot be used against them. In addition, Alexander only went East and expanded all in that direction, Rome went North, South, East and West... thier attention was divided much more around.
Of course Rome isn't that powerful, she was just starting to dominate central Italy around the time of Alexander... so you cannot compare the two. Alexander and his Macedonian Empire was at it's peak, while Rome was still a fledgling Republic...
Pyrrhus was defeated in the end by his own allies in Italy who once they won under Pyrrhus thought they did not need him and once Rome defeated them they pleaded for Pyrrhus' aid once more. His allies were just as a threat to him as Rome was and this is not including the Antigonids in Macedonia.
The Gallic Sack of Rome was in the 4th century bc, not the 3rd... which is when Pyrrhus was in Italy. In fact it was in 390bc, much longer before Alexander and before Macedonia was more than a backwater hole of little value.
You present weak arguements... besides the inheirent fact the two cannot be compared since in no way would an Imperial Roman Army vs Alexander's Macedonian Army.
it seems that i make you mado my friendo, you said rome went north, south, west and east maybe even outer space, but rome rule a tiny portion of the east not the whole eastern world, they may rule the west but not the east my friend they hit a brick wall in parthia. when they hit this wall they scratch their heads and ask each other how did alexander went further than us man..juiceist caesta