How Best to Merge Two Cat Households

I have an 8 year old indoor only cat (that is fixed) and due to a merging household may be inheriting a new, younger cat. What is your advice on introducing the two together and hopefully getting them to interact peacefully. My cat has been know to hiss and howl at cats through a window, so I'm worried that they just not be compatible. But, when I got her at the humane society she was in a large viewing area with other cats. Not sure what to do. (Paul, Portland, Oregon) 

Cats are very territorial and like to have their own 'patch'. Male cats generally want more territory than females and neutered cats want the least. Acquiring a younger cat will certainly be seen as less of a threat than an adult, although it is very difficult to forecast exactly what will happen. In most cases there are relatively few problems. 

There will be an initial period of hissing and spitting from both cats at first. The older cat will retire to glare at the younger one from a distance. It is essential that both cats have the opportunity to feed quietly and on their own. Allow them to meet periodically throughout the first few days (but always remain present) and you may find they begin to accept each others' company. This is provided the younger cat can resist chasing and playing with the older cat - which may irritate him.