[Thureophoroi] How would you use them?

[QuintusSertorius]

you answered your own question

the need for more flexibility after facing the kelts forced the hellenes to counter the kelts with their own heavy armed javelin throwing unit wich was also fast and flexible

in many ways i use the peltastai in the same way as i use my thureporoi in the end it´s mainly a matter of if you prefer spear or sword (altough in a few ambush scenarios peltastai skirmish hability can be more usefull and less usefull in a field batle against light or medium cavalry)

So in a way, that style of unit is pretty ideal if you're going to be fighting Keltoi? Assuming they're going to have lots of javelin-throwers and good cavalry.

Personally I use them as my 'grunt' infantry. Jack of all trades, they are spread throughout my battle lines, providing the bulk of my forces. In the centre, they would provide the main battle line; in the flanks they would support the specialist units and cavalry (in fact, I find them very useful in screening cavalry). Behind them is where I place other heavier infantry as reserves.

Was that the way they were used by the likes of the Seleukids and Pergamon?

Are they solid enough to be front-liners against Kelts? I have a feeling that short of hiring a lot of mercenaries, hoplite-style infantry would be hard to come by. And besides, you probably want the centre of your line to be people you trust at least some.