he looks quite powerful and would imagine if he turned he could well kill the spaniel have you tried seperating them when you feed them, i had this with small border and lurcher i stopped giving them little goodies and just kept them to high protein feeds at mealtimes nothing else to fight over and seperated them just at mealtimes it stopped the bother, someone might have a better idea or more knowledge as you say you dont know the background to the dog but at 8 mths old i wouldnt of thought it would be to bad .