Like many people, I thought that mealybugs are powdery and scales waxy and didn't know much about psyllids at all. Not so!
Mealybugs and scales are sucking insects (like aphids) that secrete a layer on their body to protect themselves and are immobile or slow moving. They are all slow moving or stationary sucking insects, that excrete honey dew from their anus. Ants attracted to the honeydew harvest this and protect them from predators. (This brings a whole new aspect to the saying "kiss my __" enough said). Excess honeydew secretions that drip down the branch are used by moulds to create a black sooty stain that can be seen on the stem and the ground below the shrub.
What we see on plants are almost all female. The adult male looks like a tiny wasp and only exists to mate. I have met some humans like that but they eat, and also live much longer. Some scales have dispensed with males entirely and produce clones of themselves (parthogenesis). Some reproduce via eggs, others by live birth, to produce mobile young called crawlers. Once it finds a nice spot a crawler develops a coat or 'shell', and some dispense with legs and antennae.
There are few easy rules of thumb to distinguish between them. This article may help.
Mealybugs are flat, oval in shape, and have a powdery coat. They prefer moist warm conditions and are not found in the wheatbelt.
Gall-inducing scales (family Eriococcidae genus Eriomorpha), produce galls (small knobby plant growths) in eucalypts Females and males have different shaped galls and the males leave theirs to mate with females. Males have different shaped galls to their female mates. This includes Gumtree scale Eriococcus coriaceous