I agree. Unless there is serious bloodshed, they will work it out. Don't show favoritism, though. There is nothing to be gained by making one of them jealous.
And they will work it out -- eventually.
It's best to match the sex of the newcomer and the established cat -- male-male or female-female. Also, it's a good thing if the new cat is younger than the old cat. The cats will figure out some kind of pecking order -- they'll do that on their own, and there's really no way you can stop it. If it requires the occasional fight, then that's what they need in order to sort things out. Break them up if things get too violent (keep a squirt gun handy), but otherwise they need to work it out among themselves.
I adopted Old Cat in 2002, and introduced New Cat into the mix in late June of this year. Old Cat was, to say the least, not happy about the whole thing. Old Cat and New Cat would get into some pretty nasty fights. Old Cat is still not delighted with the situation -- he'll hiss and growl and give New Cat the hairy eyeball whenever he walks past, but they're getting along much better. Their fights are more like rough-housing now, and it's only when Old Cat needs to reassert his dominance.
If they're indoor cats, then they have no choice but to come to some kind of modus vivendi
. As I tell my cats: "there's no Plan B." They may never be best buddies, but they'll learn to adapt to each other.