In my experience it doesnt matter how long they can 'see' each other before they are introduced they will still carry on when they are put together. They do this to establish rankings within the herd and is necessary for herd heirachy. Most of the time they sort themselves out quite quickly and get on with their grazing and pooping lol
I usually let them sniff each other through the fence to get the initial first time smell out of the way and then let the new horse into the paddock making sure the others are away from the gate and you have room to get the horse in and detach the leadrope etc before the other horses make their way over.
I usually watch for some time while they establish everything. I rarely step in and if I do its usually only with a crack of the whip if someone is getting to boisterous. Usually it doesnt take long and at the first sign of calm everyone gets some hay to munch on (at a reasonable distance from each other of course) and usually after that things are ok. Unless the new horse wants to challenge the alpha male for his position in which case they are going to fight whether they know each other over the fence or not. Its just how it is.
If the new horse is really nervous and edgy though keeping it seperate for the first day or so is fine. This is more so that the horse isnt overwhelmed by everything espcecially if its already wound up about the new surroundings. Many horses take these things in their stride but many don't. Its best to judge your horse/s and your situation and decide from there