My new addition is 12 weeks and 1 1/2lbs.. My cat is 15lbs and 8 months old.. He was hissing at him and stuff the first day.. That has stopped and now he stalks him but just close enough to sniff and run.. My big boy has his claws (I didn't want to traumatize him and get them wripped out - he had a horrible past) but he doesn't use them.. I leave all 4 of my yorkies here with him alone.. I'm not worried about him attacking them.. 4 out numbers 1 by far!
But he thinks he's a dog too ( he hates other cats ) so that works out well..