I know this question is a month old but I’d still like to answer it for you because it’s a very interesting question.

The main thing to look at which this question is where the Roman army was. The majority of finds of lorica segmentata come from the north western provinces (on the Rhine, Britain are where the majority of finds come from). It’s very very to find it coming from hotter climates due to its poor ventilation and the large metal sheets would reflect the sun into the faces of soldiers and make them incredibly warm. There is one find of lorica segmentata coming from Gamala, Israel, though this is one of the only pieces coming from the eastern provinces and currently (to my knowledge) has no publications so little is known about it. So in that sense, no not all Romans wore lorica segmentata because it depended where in the empire the army was to what armour they wore.

In places where we find lorica segmentata (Britain, Germany, Romania for example) we still have finds of lorica hamata and lorica Squamata (scale armour) which suggests that lorica segmentata wasn’t exclusively used. Bishop and Coulston suggest that due to the lack of representation of lorica segmentata on frontier monuments, e.g. the Tropaeum Traiani in Romania dedicated by Trajan after the Dacian wars, that the use of lorica segmentata was not significant and that the use of lorica hamata still made up a good proportion of the armour worn by the Roman army.

There are issues with the study of lorica segmentata too. It was very easy to break and had lots of small fittings which were easy to lose. This means that it is likely over represented in the archaeological record and past historians have likely over estimated its use in the army. Just because we find a lot of it doesn’t necessarily mean that it was used a lot, just that it was easy to break/lose parts.

Finally we come to whether or not the use of lorica segmentata was exclusive to legionaries or if it was also used by auxiliaries too. Maxfield believes that it was used to both due to finds of lorica segmentata in auxiliary forts, however Bishop and Coulston say that forts weren’t exclusively used by auxiliaries and the armour could have been from a small detachment of legionaries (such as a vexilation), and thus the use of lorica segmentata was exclusive to legionaries, meaning that lorica hamata was widely used by auxiliaries.

So to answer your question: it’s complicated and we’re still not sure on what the true answer is. It is likely that in your game most of the soldier wear lorica segmentata because that’s easy to recognise and makes them easy to identify. We see the same thing was down by them Romans, on Trajan’s Column all of the legionaries wear lorica segmentata, while all auxiliaries wear lorica hamata or lorica Squamata. This makes them easy to identify for the Roman plebs who looked upon it. But it is likely the army didn’t look like this and was a stylistic choice by the maker to make it easy to identify who-was-who.

Sources:

Bishop, M. C., 2002. Lorica Segmentata; a handbook of articulated Roman plate armour. Vol. 1 ( this is free online definitely check it out)

Bishop & Coulston, 2006. Roman military equipment; from the Punic wars to the fall of Rome. 2nd Ed.

Gilliver. K., 2007. ‘Display in Roman Warfare: the appearance of armies and individuals on the battlefield’, in War in History 14(1), 1-21.

Maxfield, V. 1986. ‘Pre-Flavian forts and their garrisons. Britannia 17 59-72.

Sim and Kaminski, 2012. Roman imperial armour: the production of early imperial military armour.