Help Introducing rats.?

[Sarah]

Okay so I have 2 RSPCA rats- Marcus black and white castrated male (castrated by RSPCA) he's a pretty big boy and Stacey our blue dumbo. They lived aone all their lives, until we got them. We've introduced them fine and there getting on well.

Now we've been given the cutest baby blue dumbo with a White nose, she was going to be fed to a friends snake who got given her for free from and accidental pregnancy, so obviously a MADE him give her to me.
So we have been doing some neutral ground into, and there cages have been next to one anothers, every week we been swapping them in each others cages, we done that for some time, all seemed fine sonwe cleared our biggest cage changed it all around etc and tried putting the little one in. Stacey took to her fine a lol bit of wrestling, but Marcus just wanted to attack (out of the cage he's fine cuddles her etc)
So what to do now... please help?!! Really want them to get along considering what almost happened to the little one.
I've kept rats all my life and had many successful mixings but all have been adult - adult
Sorry for the essay x