Eric, the two comments I would make are:
First, you won't really need an additional metric set. Each collet in the set will grip anything from its nominal size down to the next size in the range. For example, if you come across a cutter with a 6mm (0.2362") shank, the 1/4" collet will grip it. What it won't do is hold it lightly in place (as it would with a 6mm shank) on a vertical mill while you tighten up the chuck, which can be a slight inconvenience. I'm in the converse situation here in the UK, with some inherited imperial cutters, so use a 7mm (0.2756") collet to hold 1/4" shanks.
Secondly, the Glacern collet chuck doesn't seem to have any flats or tommy bar holes in its body, so you can hold it still while tightening up the nose nut. That will be OK as long as you have a spindle lock (I don't, on my mill).
Thirdly, it doesn't seem to come with the C spanner (sorry, wrench) needed to tighten up the nose nut.
There you are, three comments for the price of two!

Andy.