PEW, hooded, & berkshire feeders
It takes at least 3-4 months for a rat to become full-grown. Young rats can be apart of a larger mischief with various ages, but it's important for them to have at least one companion of a comparable age + size. Though it might not be preferable, you can easily find young rats at chain pet stores like Petco/PetSmart. I would recommend getting one and then keeping the two pairs separate until further notice, possibly permanently. A little tussling is to be expected but the level of aggression your territorial rat is showing is dangerous to your little one. While it's less common with domesticated ones, rats absolutely can and sometimes will kill each other. Also, are your rats male or female?