id people back then ever run only the chest plate with lighter armor like chaion the legs and arms mail or was it usually ran only in a full set?
“Set” is an idea introduced by DnD and RPG games.
People would wear whatever they can afford and get their hands on. Life is precious, and proper full armor costs a fortune(still does today). Poorer soldiers e.g. might have worn a padded gambeson and some ‘jack chains’ along their arms to give more protection to vulnerable areas
More well off soldiers would wear a brigandine armor. Which is lightweight and provides excellent defence and mobility.
Here’s a photo of an underside of a brigandine to give you a better idea what it is - it’s a ton of small metal plates riveted onto linen or leather ‘shirt’:
and on the outside you would see rivets and rich textiles or leather, sometimes with simple or elaborate decoration for overwhelming beauty
It might have been worn over gambeson or over gambeson+chainmail, while hand protection could have been plate or those jack chains or a combination of.
In the end - in KC:D even when you mix and match - if you like how the result looks you’re probably not far off from an ‘ok’ estimation of what a man at arms might have looked like then. This is not a re-enactment event where you’d have to prove thar you costume is accurate, enjoy the game
If you want to ‘fit in’ pay attention to how guards, bernard, robard, etc are armed and dressed, they all follow this.
E.g. padded chausses for leg protection, some sort of boots, gambeson, brigandine and brigandine pauldrons + composite gauntlets. Then chain ‘scarf’ to protect neck, padded coif(hat\hood) to pad the helmet, and then either helmet directly or chain coif and then helmet. Something like that.