if you use 5/16" or 3/8" tubing bought from a chandlery you'll be OK. appliances, regulators, bubble testers, etc. generally have imperial threads so the fittings that are used with imperial tube will also match the appliances.
you are unlikely to find tube of adequate wall thickness in metric sizes. regardless of whether it is legal, it is simply the most difficult route to choose. you would save a bob or two on the tube and suffer heartache and extra costs trying to get it to fit at the ends.
no more confusion.