stick cat in mailbox face first insert stick of tnt underneath, close door,
leave quickly, no more kitty with 'tude

I had a similar situation a couple years ago.  Place the new cat in a room
by himself but leave the door open a crack or better yet, get some cheap
baby gates and put them up in the doorway of his room.  That way the 2
cats can see and smell each other without causing harm to each other if
they were so inclined.  If after several days the hissing and growling
(which is natural) calms down you can try to open the door/take down the
gates and let them have supervised time together.  Try to make it
positive, I used tuna fish as they both loved it.  give them each some
tuna when they are next to each other near the door/gates and then when
they finally come out give each tuna when they are in proximity to each
other. Praise each one when they dont hiss or growl at each other.  It
took me a couple of months but now they can live together although my
older cat (she's 8) doesn't really like the new one (who is 3) they do
tolerate each other with no fighting.  Just be patient.  Good luck!