This is the same with plants and animals. Plants are grouped together in families and animals in classes. If you ever have a change to look at the flowers of these species you will notice that they do have a lot in common. The seed capsules look even more similar and all these species produce very small fine red coloured seed that almost looks like short fine fur. By the way, Eucalyptus, bottlebrush, and feijoa trees also belong to the Myrtaceae family.
Families are groupings of species with similar features.
Usually more than one feature is used to identify which family a species belongs to. It might be the way a flower looks, what sort of seed the tree produces and may be even leaf shape. Lazy scientists Scientists are just as lazy as other people and get rather bored writing out full Latin names all the time. So, the convention is that the first time a species is introduced in a document its Latin name is written out fully. The next time that this species is mentioned the genus name is shortened to the first letter.