First off, this is the incorrect forum for this query. I've notified a mod, and since you're fairly new, they
should move the thread rather than locking it.

In the future, most of the subforums have a sticky at the top that tells you what threads are allowed there. This forum is for rules disputes only.
Secondly, scarabs work like normal swarms work. Beasts, ignore difficult terrain, no ranged weapons, not really great in assault, but really hard to kill. In addition, they have the entropic strike rule, which means that they can really mess up the day of monstrous creatures or heavy vehicles, since they can strip their armor away.
Usually, I see scarabs used as a tar pit unit. They charge into my elite units, keeping them from doing anything but squashing bugs, usually for the majority of the game. If a swarm of them gets ahold of my Land Raider, then it's usually gone within a single assault.
The only way for them to multiply is in the presence of a Canoptek Spider. The Spider can generate new bases to be added to the swarm, at the risk of harming himself.
Hope this helps!