I've experienced both the good and the bad with introducing males. What I've found works are a few different techniques.
First method I've tried successfully is filling a spray bottle with vanilla extract and water. (buy the big bottle of vanilla extract). I've sprayed the ratties down, put them in a smaller area (i.e. carrier method - I think Isumurat has a fantastic article on her site about it), and monitor them. The rats are so engrossed in grooming themselves, and then each other that there was very little scuffling going on. Downside to this is that any white marking on your ratty is going to get stained from the vanilla. I usually keep them in the carrier for 14 hours, and in the beginning keep them "vanilla-fied".
Another method that has worked for me when the Vanilla doesn't is to give the rats a bath using baby shampoo. I just introduced a gruop of 5 rats, 3 are 7 months in the beginning of February, and two are a couple months younger. I started with my most dominant rat, gave him a bath, rinsed him really well, and then put him in the tub, which was filled with a couple inches (at the deepest end) of warm water. Second dominant rat bathed, and also placed in the tub. Then one of the new guys. I had a very fearful, reactive boy who freaked out anytime anyone look at him. He was washed (took my time, moving slowly and gently with him as he was petrified), and then placed in the tub. Since I had added the fearful boy I monitored the group for 10-15 minutes. They were mainly concerned with getting OUT of the tub, and less interested in each other. Once they stopped trying to get out of the tub, they paid attention to one another, but none of the usual dominance stances were happening, so I continued bathing the last two boys (one used to the group, and one new boy). I had had several failed attempts at getting these guys introduced successfully - to the point of blood being drawn a couple times. All five stayed in the tub for probably a half hour. I had a big tote modified into a birth box ready with fleece in the bottom, and began pulling the boys out one by one and drying them. Each went into the tote, and they spent the next 24 hours in the tote together. By the following evening they were a bonded group and TB (the very fearful boy) was smack in the middle of a rat pile. I had no trouble reintroducing them back into the DCN with the rest of their cage mates and since then (couple days) have had no issues whatsoever.
Good luck with your guys! And if you go the route of the bath - be prepared to come away with a LOT of scratches. Three of my boys are super docile and were easy to bathe, but the two new boys were set on getting AWAY from that water... which mean climbing up my arms.