Hello Farhan, Welcome to SD. You will find a lot of help and support here. I'm kind of in and out but noticed your post and I am not an expert but have had similar issues with aggression and egg layers.
My tank is only 75 gallons and I have ten discus in there. yes, I am overstocked. It a long story and I change a lot of water. I will start by saying adding a few more should help disperse the aggression. I will add though that before you do you should really consider getting a quarantine tank. You've been really lucky so far with the addition of new fish and no illness. If you plan ahead your QT can become a breeder tank at some point if you do decide to try the pair. I'm one that said I would never breed. The thought is a spark that is still alive in there so you just never know. It's definitely a must have especially with a planted tank of your size. One sick fish and treating 120 gallons can get to be prohibitively expensive. So in short, I would get a few more and QT them first.
I'm assuming much of the aggression is stemming from the egg laying activity. My pair will protect their corner like pit bulls until the eggs are gone and then mix nicely back in with the group. If I keep my water conditions up they will continue to lay eggs every 10 days to 2 weeks or so. I wish they didn't. It takes a lot of energy out of them and it's sad to see how hard they are trying but they just continue do it.... During the aggression periods is it possible to target feed the others?
I find that small changes in the tank will disrupt the order of the group. If one is taken out for QT or if I even rearrange the driftwood it starts all over again. It's just the nature of the cichlid beast. Once in awhile if the aggression gets to be dangerous and turned onto one specific individual I will tape a couple of 12"x 12" mirrors to the tank to give the illusion that there are actually more fish in the tank. It has worked for me each time and i remove them as soon as things have settled again.
Just my own thoughts, others may have other ideas. I hope things settle for you soon. Welcome again and enjoy the forum. There is so much to learn here.