Arms and armor and clothing was never standardized by the Romans, in the sense it was uniform. They had a few qualifications in terms of what it needed to consist of, and if it did, they bought it and issued it.

By the Late Republic, especially during the Social War and Civil Wars, when a large part of the eligible male population of Italy ended up serving, quality of equipment likely dropped, designs and construction methods needed to be simplified. This is likely why the Coolus type helmet, basically just a bowl, without cheek guards, often without a neck guard, replaced the more elaborately designed helmets of the Mid Republic, like the Montefortino and the Italo-Corinthian.

Additionally, not everyone wore mail armor, some likely didn't even possess corselet torso body armor. Other types would include various cloth or leather types, pectorales plates, scale armor, etc.

It would not be until the Empire, with the legions under the command of Augustus and subsequent emperors, when things became more standardized, with less wars, less soldiers, and ease of supplying/issuing.