Yes, assign them specific armor. Instead of metal breastplate, you got to choose, which specific breastplate they'll wear. It's a hassle to assign all that stuff to every military dwarf, but once you have done it, they will never unequip their stuff again, when your armorsmith has again "created a masterpiece!".
not sure what you mean. can I go even more into detail than telling them which colour and which material? can you choose from a list of available armor pieces somewhere?
Specific Armor when assigning equipments, have to do it from equip screen instead of uniform screen.
As for your original problem, I don't know what's happening, are they set inactive = uniformed in 's'chedules? Sometimes during month's end or season's end, they'll inactivate if their military skill aren't high enough, for a moment, then reactivates, and I've not had any issues as long as it's uniformed, instead of civ clothings. Or are you using like iron and copper equipment and dealing with goblins that brings in iron armors? Their equipment decision is made purely based on value and on what's allowed in their uniform.
Forbidding stuffs doesn't helps once it's been assigned, they'll just not equip it. Forbidding things does helps with equipment assignment or for fresh uniforms, though, it just don't works if the equipment's already assigned but not picked up yet.
Is the uniform set to replace clothings? Sometimes there're too much stuff to put things on properly, which part of armors give the most problem? The helmets and the boots?