How long has Topper been in the house? If it's been over a week, go ahead and try the new cat in a carrier and let the other cat sniff a bit - this is part of the introduction protocol. Guaranteed it won't be a long "visit" -- I think my two lasted about 30 seconds before the hissing and growling started at first, I increased their visit time each day and after about a week I let them visit without a carrier -- the carrier was too stressful for both of them anyway. Good luck! This is the hard part. It does get easier, but it takes time -- kitty time!