At the risk of getting sued for Copyright infringement I am posting sketches of the troopers of the more important units which formed Alexander's Army:
The first is a Standard bearer and his "kopis" can be made out in the sketch.
The Second is a sketch by Barker of a Macedonian lead rank Phalangite showing how the Sarrissa had to be held and the Shield lashed to his arm.
Sketch 3 is of one of Alexander's unparalled light infantry, or "Hyspapists". Alexander's light infantry cannot be appreciated enough and were the only force able to deal with the problems in the Afghan Mountain before US special forces did .
Armed with Xyston (a weapon slightly reminiscent of a Roman Lancea), and Kopis, lightly armored, unecumbered with the sarissa, he went where he was needed, and went there fast.
Of any of Alexander's infantry, the Hyspapist was most likely the best trained with the Kopis, and a dangerous opponent, his elite status second only to that of the Soldier shown in sketch 4, the "Companion Cavalry Man", also armed with Xyston and Kopis, well mounted, exquisetly trained and more heavily armored than most Macedonian troopers of Alexander's army.
The Great Sarissa armed Phalanx, typically disposed on Alexander's Right, would 'fix" the main mass of the enemy and the Companion Cavalry would charge home, usually at the right juncture of the Phalanx.
Alexander's left flank (under Parmenion in Persia) would have been made up of more typical Hoplites bolstered by "Peltasts" and perhaps Hyspapists.
Well, since the Peltasts were no more than expert rock chuckers, you can guess where the English word 'pelt' may have found a root. The Peltast was relatively lighty armored and protected by a crescent shaped shield.
By the time of Issus and Granicus, Darius had tried to bulid a corps of forces based on the a "Persian" Hoplite, but I have found no record of how effective the ersatz hoplites werer at Guagamela, yet.
It seems always a mistake to try and defend every inch of ground of your territory at your border, as Darius tried to do, against an opponent like Alexander or even Phillip. Although Darius' choice of ground at Guagamela was certainly well made.
Otherwise Darius had a penchant for fighting before he was ready and it cost him the losses at Granicus and Issus. These losses had a large effect as I believe Darius' best trained troops, other than Bessus' cavalry, may have been decimated in those frays.
More on those later..
Having wargamed Guagamela, it still appears Darius might have won with a more judicious disposition and application of his own elite cavalry and infantry.
Perhaps a less unimaginative battle array would have served him better, but Darius' army came from many nations and had stayed up all the night raising a great clamor to effect the morale of the Macedonians, and was, at best, probably hard to control.
Now, I must say this was not a good move. Darius' host was not close enough to have the desired effect on Alexander's men--and good soldiers learn to sleep through nearly anything.
Bessus' Cavalry was superb, but the whole of the Persian Cavalry Corps seems to have gotten away from the archtypical winning formula of the "Horse Archer" that had led the Persian Army of Cyrus to conquer the Eastern Mediterranean.
Note the Phyrgian (I call them 'snoopy') helmets of the Hyspapist and Phalangite, which can sometimes help to differentiate between the troopers of Alexanders army and those of the Successors.