I never really follow the rule of keeping them separated. I just go on the cats behavior. When we introduced Ramsay and Gus they hissed for a few minutes, within an hour they were playing together, and by the next day they were sleeping together and grooming one another so keeping them separated was completely unnecessary.
When we introduced our cats to my in-laws cats we put ours in a room with their food and litter for a day or two, then we let them mingle with one cat at a time (they have 3) so it wasn't too overwhelming and kept a close eye on them. The plus and minus to that method was that our cats established that room with their litter and food as their territory. It was good because they could use that space as a retreat or a safe place to go nap. It was bad because when one of the other cats went in there there was occasionally major problems. One of the in-laws cats came in the room and ended up getting into a fight with one of ours, and at another time one of the in-laws cats came in and peed on some of our things.
So, my advice is to go on what you think would be best for your cats. You know them better than anyone here. If you think your boys are gentle or submissive enough to be good with another cat coming into the house, just try introducing them right away. If you think there might be problems, separate them. If things are iffy, keep them out together when you can watch them, and separate them when you're not around.