Not always so a lurcher will kill a terrier yeah but two terriers locked in a pen if they go at it the result can still be the same,there is not set rule for anything otherwise we would pen all dogs separate in 20 years ive had 3 gripes in kennels and all 3 from terrier penned with terrier never had 1 bit of trouble lurcher and terrier penned together,now that could go wrong at anytime my lurcher could kill my terrier tomorrow ye never know but i usualy pen dogs together on my judgement of there temprement