Using connectors rated for 250-400V with 5V
Connectors are typically rated with a maximum voltage, meaning every voltage up to the rated one is okay. For you that means the 12V and 5V are totally fine to be used on a 250V connector. You still need to make sure you don't overload them with the current you are going to use, e.g. a 5A/250V connector should not exceed 5A, even if you use it only with 5V.