Good advice has already been given about going slow with integration, but I wanted to address the shaking, biting actions you described...
In my experience it is not actually an aggression sign, good or bad... I think @Tevyes Dad is the one who called it the "I'm gonna do something crazy duck style" type sign...
From what I've seen, it's more of an excitement reaction... I've seen mine do it when they were presented with a bowl full of peas and they were trying to decide if they should eat slower an enjoy them or gobble them down before the others could eat their share...
I also had a Call that hatched out singly, and a week later a pair of Mallard/Rouens hatched out... the Call went all bity crazy and I had to keep them separate... a couple weeks later I had a pair of Calls hatch out and she did the same with them... so I had 3 'groups' to keep separate... once the Mallard/Rouens got 4x the first Calls size, they tried to pick on the pair of Calls and the first jumped to their defense and proceeded to accept the 2 as her flock...
All flocks have a pecking order, and when they're of a size to not be injured easily, they will work it out... we just have to watch out for those little ones, cuz they can get seriously hurt by bigger ones acting as if they're all the same size and age...
Hope this helps...
Edited cuz I can't spell on my phone, lol...
Thank you so much for your info. I have experience integrating new chickens to the flock but not ducks. I'm hoping it's about the same. I will be slowly putting these 2 pekins in with the other 6 I have in about a week when they are fully feathered. I hope things go ok. The babies still have many weeks before they will be integrated but they love to come outside and sit in our laps. They have extreme anxiety when they are not with us.