[rotrex]

Don't drill, just buy the specific pipe adaptors from metric to imperial sizes.
You get them as simple plastic push lok, metal push lok up to the most expensive Swagelok stainless fittings with ferules all in 316l stainless steel.

Alternatively, buy a FAV and see if you can swap the 6mm fittings for something in imperial sizes. As a injection system needs several fittings anyhow, it should be no problem to go from 1/4" to 6mm for a FAV and back to 1/4" for you injectors shall you prefer that.

Aquamist makes excellent nozzles. I have used them for may years with excellent results. Never had one fail. You even can open them up to clean the internal filter frit and inpect the nozzle hole from both sides.