It is not very fruitful to compare the written sources about Alexander to those about Caesar.
Caesar happened to live in the midst of a time every serious man recognized as one that was to change history. Hell, he recognized that himself! That's why he wrote down the Gaullic and Civil Wars. And despite the obvious tone of self-praise, his style is surprisingly sober and realistic.
The authors who wrote about him -Cicero, Suetonius and Plutarch to name the most important- lived in the same civilization as he did, so they had a clear view of things. This is why we can consider the written sources about Caesar trustworthy.
Alexander on the other hand lived in a strange time. For some reason, no historian can be placed in this time in succession to Herodotus, Thucydides and Xenophon. Of course companions of Alexander wrote- and they didn't write very little. Callisthenes was Alexanders 'personal' historian. Soldiers wrote of their experiences. So did Nearchus and Ptolemaeus. But for some odd reason, nothing of this survived in a complete and unedited way. There are five complete histories of Alexander: that of Diodorus (ca. 50 BC), that of Plutarch (ca. 50 AD), Arrian (ca. 150 AD), Quintus Curtius Rufus (150 AD) and Iustin (ca. 250 AD). These often quote from their sources, but between Alexander and these historians lie centuries, in which Alexander became more a myth than a memory.
We can rely on a certain extent to these historians. But then, all of written history is reliable only to a certain extent.
I would also give an opinion to Parmenion's statement that the Romans copied off the Greek civilization.
Throughout history, you will find many civilizations that bear the one or other astounishing similarity to others. Just look: Persian bas-reliefs look oddly much like Assyrian and Babylonian ones. Carthaginian cities (though lost) had very much the structures of Greek ones, and Roman temples looked oddly much like Greek temples as well.
This is not a question of copy. It is a question of influence. Many civilizations regarded foreign ideals as true and good and adopted, assimilated them (a very widespread practice, for example, in Hinduism). That doesen't mean they copied foreign civilizations. Yes, Roman architecture, at least in the earlier times, bore remarkable similarities to Greek architecture. How could it have been otherwise? They were surrounded by Greeks, and also by Etruscans and Carthaginians, who had also adopted Greek ideas. They knew nothing else but tribal cults from their origins. In order to establish a civilization to challenge the others, they had to adopt certain things. And by the time they became unnecessary, they had already become so deep a part of their culture that they didn't consider dropping them again, but advancing them.
This is not a Greek feat. The same has been accomplished by the Babylonians, Assyrians and, yes, Persians, and so many others.
There has been a time of imitation and copying of parts of foreign civilizations. But this was in the Rennaissance and later. That again, had a totally different cause.