Skin Problems Lice. Ivermectin or Adavntage?

Both of my guinea pigs have visible very tiny yellow/clear almost "pill" shaped things all in their fur. Also some dry flaky skin. I'm assuming from what I've read, since they are visible, this is lice and not mites. Although since these things seem to be tinier than most pictures I've seen I'm not too sure. How big are lice usually?

Is the original Advantage still available? If not, can someone walk me through giving Ivermectin to my guinea pigs? I know it can be dangerous.

Maybelline's weight is 1404g
Penelope's weight is 1033g