ISO-6432 air cylinders ; some of the North American manufactured throw-away - non-repairable - type air actuators and cylinders are not manufactured to any known, industry-wide standard.
For example, if you were to purchase a 3/4" bore size air cylinder with a given stroke length from one vendor, there is no guarantee that if you purchase the same bore size and stroke length cylinder of another brand that they will be interchangeable in terms of rod diameter, port size, or mounting footprint.
On the two air cylinders, thread diameters and rod threads may differ, mounting footprints may even completely change, in short - if you do not buy the same brand again - you might have to alter how you mount the air cylinder, and how you connect it to the work piece to make the new cylinder work in your application.
ISO-6432 air cylinders, regardless of what company manufactures them, will have standard critical dimensions.
An ISO-6432 non-repairable air cylinder from a different manufacturer may not look the same as the one you are trying to replace, but the new cylinder should fit the same footprint as the old one.
This brings a higher level of complexity to the workplace where a non-repairable cylinder may have North American roots and Imperial dimensions are the norm.
If you are trying to replace a non-repairable air cylinder of North American manufacture with an ISO style (they may even look similar) expect that you will have footprint and thread issues.
NPT fittings normally do not work in metric ports, though some fitting manufacturers suggest that they have a universal (NPT and Metric) line of air fittings. Here is more general information on air fittings.
When you have located your metric fittings for the ISO 6432 cylinder, the air line tube sizes that normally fit those metric fittings will typically be 4mm, 6mm, 8mm and larger tube. Air line tube is measure on the O.D., so a 6 mm air tube will have an outside diameter of 6 mm.
The 1/4" P.E. tube that is almost the standard for machinery in North America, will not be usable in a metric fitting, unless - and only perhaps - the part of the fitting that connects to the tube is a barb. In that case, you might just be able to connect a metric fitting to a 1/4" P.E. tube. Good luck with that!