If you have a 1/4" SS line it is probably seamless tubing with compression tube fittings which means that a hydrotest is useless sense hydro's are for testing weld integrity only. With a harmless fluid like helium a simple leak test at service pressure ought to be adequte.
GTD
Well, I have seen every possible response to the old 5 fps rule, except for the one that I learned. I did the piping design for a hotel and the architect told us to keep the velocity to 5 fps because one cannot hear the sound of the water rushing through the lines, which, if you think about it...