Males are a tad trickier than females...particularly if they are around 6 months (when most males go through a stage of aggression)...my guys are about 7 months old, they can hardly tolerate each other, I couldn't imagine introducing them to another one lol.
A neutral area is always good...and a drop of vanilla on all the rats being introduced works nicely, too. And always remember: no blood, no foul. Let them wrestle all they want, unless anyone starts to bleed. Introducing a young rat to an older one is one of the easiest 'pairings', too.
Here's a prety good article on intros~
http://www.ratfanclub.org/newrat.html