Sadlt Tael,when,he was a stray,may have had some bad experiences,with Dogs,usually as a result,of some idiot,siccing,the Dog,on a homeless Cat.Which,is stupid,as Cats,at least at first,have the advantage,of speed,and claws,and the Dog,gets hurt.Being fixed,should help,and we hope,that as you say,they will become Friends,if not bosom Buddies.