"напугал кота сосиской" seems to be an idiom. Is it? What does it mean?
Yes. It's an idiom. It means "to scare a cat with a sausage" - a ridiculous threat.
There are some variations of this, like:
Напугал кота сметаной
Напугал кота котлетой!
"trying to scare a cat with a piece of steak"
Also some other animals are involved to in the similar pattern and meaning:
Напугал кобеля буханкой
Above animals do like a food mentioned, but a hedgehog just doesn't care, isn't afraid of:
Напугал ежа голой жопой
"[you are] scaring a hedgehog with a bare ass"
напугал козла капустой
напугал козла огородом
напугал барана новыми воротами
The same attitude can be expressed by a bit different but close analogue:
не так страшен черт, как его малюют
In addition to many examples that Ruslan Gerasimov has made there is
Только не бросай меня в терновый куст.
A phrase which comes from a folk tale, where a rabbit begs a wolf who has captured him, to do whatever the wolf desires except throwing the said rabbit into a thorn bush. Once the wolf throws him there, the rabbit quickly runs away.
Unlike the other variants, which signify the futility of the action, this one actually baits it.