That doesn't sound like a promising start...
My first question comes from the quote "He sank and lodged himself in a rock"
That doesn't sound like normal behavior for any fish let alone a new addition..
Second thought is, "hermits eating his tail" also doesn't instill confidence... A healthy fish, new to the tank or not, wouldn't allow this behavior .
I have many questions, and none of the above instills me with confidence...