Thoughts and questions...

[anubis88]

No, i don't believe that was common for the Diadochi armys.
This experiment of Antiochos failed, this army arrangement was one of the main reasons for losing the battle. The elephants fled, and the phalanx suddenly had giant holes in it.
The Romans did great damage to the phalanx just with their pillas, but after the elephants broke the battle was lost

[I of the Storm]

That was my suspicion too, thanks. Technically, with the creation of these joints between phalanx you also create possibilities for gaps, thus additional flanks. It was probably worth a try though. Although the following peace treaty was rather devastating...

[konny]

Pyrrhos is said to have used a "light phalanx", that is phalanx mixed with Thureophoroi and the like. In another post I had compared that to the Spanish Tercios of the 15th - 17th Century, even thought the Tercios (or better their pike formations) did not act as a single line but more as moving fortresses on the battlefield. In consequence the musqueteers attached to it were called "garrisons".

[Watchman]

Well the major problem with the pike phalanx even on level ground tended to be that because different sections pushed back the enemy at different speeds, the sub-units became decoupled from one another and the line became ragged.

Pyrrhus' "mixed order" was presumably an attempt to get around this by accepting the disjointing as inevitable (especially given that the Romans pretty much flat out refused to fight the phalanx on level ground), and providing the pike line with organic lighter-infantry support to guard the sub-units' flanks.

Pretty sure the main problem of the Seleucids at Magnesia was that they lost the flank battles, though. On their right wing the Roman skirmishers drove the scythed chariots right back into the Seleucid horse, and a massed charge by the Pergamene and Roman cavalry did the rest. On the other wing the massed Seleucid catas apparently pretty much stomped flat everything before them, but then instead of turning on the Roman center inadvisably tried to pursue their defeated initial opponents with the end result of becoming exhausted and hopelessly disordered, so the Roman counterattack drove them out of the field.

Which sort of left the pike phalanx in the center hung high and dry, and the Romans mainly just surrounded it and let their missile troops shoot it to bits before the heavy infantry mopped up the remnants.