Babies are sometimes too fragile, too small, or just too silly to cope with an aggressive older rat. But by 8 weeks and certainly 10 they should be able to defend themselves adequately without being so aggressive that they pick a fight with the older rat. Note that since male rats are often not aggressive to baby rats, it is better to try to introduce a baby boy to older boys as soon as possible and NOT to wait until he is old enough to stand up to him. But if the older boys seem vicious towards him then you may feel that it is better to postpone the introduction. These suggestions have been gleaned from various sources and numerous rat owners with far more experience than myself.
Rats tell whether other rats are part of their family or social group by smell.
If you can make your two introducees smell alike, or at least disguise the smells with which they mark their territory, then they may not be so defensive towards newcomers. Dab each rat all over with something smelly like aftershave or perfume. Some people use vanilla essence because it is non- toxic, won't irritate the skin, and some rats like to lick it off because it tastes nice. You should also make sure that the rats are introduced on neutral territory which doesn't smell of either one - if this is a cage, it needs to be thoroughly scrubbed to remove all smells of the previous resident. Some people suggest putting each rat in the other's cage in turn before you introduce them, so that they get used to each other's smell.