I received a letter from my previous employer about missing uniforms. They have given me 7 days to either find the lost uniforms, pay 600, or court

I'm not sure why you would think that your former employer could not send you a letter asserting a claim for stolen property, but it can.  Obviously this doesn't mean that it will follow through on suing you, or that it will succeed if it does, but there's nothing wrong with the letter.