In general the size of a needle should correspond to the size of thread, not the type of design. I use a size 11 for just about everything since I generally use a 40 weight thrad. I also have some size 10s that I used for an heirloom 50 wt thread on fine batiste. The pro machines have different needles than the home machines that last much longer (a few hundred thousand stitches), and I still go through a LOT of needles. I buy in bulk and just buy the 11s. I even use a 11 on thick quilted fabrics. For metallics, I use Yenmet and I have no troubles with it and an 11 either. Some metallics are more fragile though and might need a bigger eye.
Pam