Ancient army numbers

[Paralus]

And it is important that Arrian did think that these casualties were substantial, not average, for a melee action that involved 2,500 Hetairoi among others.

How do you see 2,500 hetairoi (Companion Cavalry) at Gaugamela?

The battle of Gaza saw many cavalrymen killed - as many as 500philoi of Demetrius if the cavalry-centric description of Hieronymus is correct. This from some 2,900 on this wing. Now, it won't do to argue that this is Macedonian against Macedonian because that is not correct ("no less than fifteen hundred horsemen of all kinds"). The losses accorded to to "the Great's" cavalry are court pap. Clearly, at Gaugamela, the Macedonian cavalry lost more than those recorded; Arrian's narrative says as much.

Arrian 3.15.2:
...and Hephaestion himself, as well as Coenus and Menidas, was wounded.

Diod.17.61.3:
Hephaestion was wounded with a spear thrust in the arm; he had commanded the bodyguards. Perdiccas and Coenus, of the general's group, were also wounded, so also Menidas and others of the higher commanders.

Now, we may argue about Hepaestion, but Heckel is surely correct that he led the somatophylakes (= agema of the hypastists). Even if not we have two certainly attested infantry leaders in Coenus (the asthetairoi) and Perdiccas who are wounded in this "skirmish" of cavalry.

I remain to be convinced George.