I've always found a greater problem with separation of the paints constituent parts when using coloured metallics. If you return to a paint that has separated what you'd be putting on to the mini' could look different to the area you last painted so it's something to be mindful of occurring.
The metal flecks don't seem to want to settle as much as the coloured parts so your palette may need a brush wiggling through it a few times, or be prepared to work on small areas at a time with fresh paint.
I have enjoyed blue metallic on elves before. Way back, when GW had coloured metallics, the 90's I think.
Can highlight with whatever silver or pure metallic medium you have added to the base coat. If you're planning on spraying them with a matt varnish, be prepared for it to dull the metallics. To the extant it may be worth saving the final pure silver highlights until after the varnishing is done.
Shading on metallics works best when the darker areas aren't just darker but have less metallic flecks reflecting light too. Can either get a mix of the metallic and black in the shadows, or just dull the flecks down with some ink layered on top.
Exclusively using blue as the shade can make it start to look cartoony. As can pure black, which can also look dirty. A mix of blue and brown has served well when going for a gritty realism style.