Introducing a mature rooster to a flock of mature hens (or a flock with some mature hens) is usually the easiest integration possible. The rooster struts in, wow's them with his magnificence, mates a couple, and it is his flock. Occasionally the dominant hen objects to giving up her flock master position so it may get violent between them for a day or two. Usually when that happens it isn't that bad but I consider it your area of biggest risk. It can get pretty violent.
The question is "How mature is that 7 month old cockerel?" I don't know. I had one that was mature enough at 5 months. I had one that did not win them all over until 11 months. Most of mine are ready at 7 months but that can vary a lot.
Another part of the equation is the hens. Some will squat for practically anything in spurs, even if they are just tiny spur buds. Others may resist a fully grown mature rooster. I think that had a lot to do with why that 5 month old cockerel was able to take over and why the 11 month one had problems. A big part of that is on the cockerel and his self-assurance but part of that is on the hens.
I don't much like the idea of locking them in together overnight and hoping things work out in the morning. A lot of times integration goes really well, whether that is rooster to hens, hens to hens, or even chicks to a flock. Especially if you have room, it really can be that easy. A lot of people on this forum don't have that kind of room so it may not be that easy for them.
One way chickens have learned to live together in a flock is that when they have conflict the loser runs away. There may be a bit of chasing but usually if one does run away and can stay away they work it out. If one cannot run away the winner does not know it won and keeps attacking or decides to kill a rival. If the loser gets trapped against a fence or is in too tight a space to run away it is in great danger. My preferred method is to introduce them during the day so I can observe and they have as much room as possible to run away. If you try the after dark thing (which often works) I'd be down there first thing in the morning at daylight to see how it's going.
Any time you introduce another chicken to your flock you run the risk if disease or parasites. Many flocks have flock immunities, where they are infected or infested and are carriers but since they are immune they don't show any symptoms. Coccidiosis is a great example but there are others. It could be your flock infecting the newcomer. Quarantine doesn't do much for that. If a chicken has been exposed to another chicken in the past month quarantine is a great idea, that's what it was set up to handle. The way I look at it, if ("if" is a big word) but if the flock he is coming from has been isolated for over a month and the person taking care of them would recognize a problem if they saw it and was honest enough to tell you they have been in quarantine, just not under your eyes.