IRL: Chain armor converts a cutting edge to a blunt collision. Normlly you'd wear "padded armor" under it, as you would most of the other armor types.
I tend to divide armor into two categories: Soft and Hard.
Hard armor is like a hard shell. It blunts and distributes blows. Soft armor just blunts, but doesn't distribute.
In my mind the soft armors are Chain Shirt, Chain Maille, Padded and Scale. (Scale armor is scales joined by steel links, so it's almost a form of chain.)
Hard armors include Leather because leather armor isn't the same as a leather jacket. It's hardened by boiling it in oil to drive out the water. The result is a material called curaboli. It's soft and mouldable after boiling and before it cools, then it gets stiff and hard. )To do this with modern materials try soaking your "armor" in water laced heavily with Tite-bond wood glue (yellow-white stuff from the hardware store). Shape it over a form and let it dry and it will be stiff as wood.)
A breastplate or set of plate maille that isn't made for you is like trying to dance a jig in one of those inflatable sumo-suits. Except more painful. If the hard plate is an inch too wide for you your arms will hit it when you try to swing a weapon. You'll develop a chronic bruise known as "armor burn". If it's an inch too narrow you can't take a full breath. If it's too long in the torso you can't even bend enough to sit down. Your legs/lap will force the plate up under your chin. (Gods help you if you need to retrieve a dropped weapon in combat.) That's why the rules call for a skilled armor smith to modify captured plate maille.
The difference is, I'd require it (at a reduced rate) for breastplate, splint and half plate as well. You're trying to shape hard shell armor to a soft body. (Note I say "I'd require" not "I do require". Overall, I'm a "do it by the rules" guy, even when I know the rules could be improved.)
As for custom magic items: D&D has a great mechanism for limiting custom made items: Exp cost.
I know they've eliminated that in Pathfinder and 4e, and probably in 5e, but it serves a purpose. An NPC craftsman only has so much "creative energy" available, whatever he can earn through non-combat challenges encountered in normal life. Since local nobility likes their goodies, in just about any major city there is what amounts to a waiting list for such craftspeople. The rarer the craft, the longer the list. Magical jewelers (those with the Forge Ring feat) are the rarest (12th level, minimum, to get the Feat), and so there are fewer in any given city. They never lack for business.
Now enter the PC, a stranger in a strange land, with heaps of gold and little time to spend it. Seldom will they want to wait their turn in line. In some cities they may not even get a place in line. So what do they do? They curry favor from a local Lord, High Priest or Guild master, somebody who will trade places with them in line, or will authorize them to have a place in line. All for a price, of course. Typically a job they need done, though sometimes for some future service.
Yeah, I use this as a plot-hook generator. It involves the PCs in the local situation, builds them a network of allies and rivals, and gives me an easy way to hand them a mission.
Sometimes they volunteer to do a favor for a particularly powerful noble or high churchman, even when they don't need a favor. Nice to have a few banked for that rainy day, you know?
It also helps regulate how quickly the PCs can convert raw wealth into magical power, which isn't a bad thing.
