Should I choose 125.5um or 126um hole ferrule for 125um fibers? What is the difference?

126um is the recommended ferrule hole size for 125um singlemode fiber. There are some 125.5um ferrules available for critical alignement situations, but for general connectorization, the 126um hole allows for some epoxy bond around the fiber, and for ease of fiber insertion without breakage.

You can choose from a range of connector products on the Connector and Mating Sleeves page.