1.) Didn't hypaspists fight on the right flank of the battle line, as the right hand of the phalanx? If so I would say they needed more armour against phalanx armies. Could be wrong though...

2.) You must have mixed up the units, for pezhetairoi are "a high class phalanx who are also well equipped and trained for melee".

3.) The argyraspidai were the elite corps of the pezhetairoi.