Sorry to discover your thread so late. I do not usually visit this part of the forum.
It is truly sad to hear you have lost your guinea pig in such cruel way. 
Are you sure it was the doing of rats? It could be other animals. For example, a raccoon, or even a cat. It is unusual for rats to hunt something bigger than themselves or even animals compatible in size.
When you have pets around, it is not a good idea to leave rat poison around. It can backfire. As for traps, you can try.
If I were you, I'd set up a surveillance camera or something like that to monitor the activity in the garden overnight to find out the real killer.
Meanwhile, it is a good idea to keep your pets inside your house.
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