We're not entirely sure, given not a lot of historical focus was put on the allies. It's assumed they fought in a manner and with equipment broadly similar to that of the Romans. Which would make a lot of sense given they fought alongside them.
I personally use the Second Punic War as the signal that it's time to stop recruiting traditional-styled Italian units (Bruttians, Lucanians, Samnites, Campanians). Many of these communities defected to Hannibal, and thus weren't trusted with independent recruitment after the end of the war. Instead they were recruited in the Roman fashion.
In my Roman games, I have a series of "reforms", some of them roleplayed:
-Polybian Reform - usually around 240BC
-Change of Italian Allies - usually around 210BC
-Gracchan Reform - around the 130sBC
I haven't actually had a game survive long enough to get to the Marian reforms; my last one was killed by CTD in 133BC just as I was enacting my Gracchan reform.
I "provincialise" the provincial armies over time anyway, with the Italians being replaced by native troops. Usually at by changing the Left Ala for natives, and keeping the surviving Italians for merging into the Right Ala.
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