Well... it depends on how much larger the big rat is. Sometimes, an adult rat will kill a foreign baby rat. I've seen it happen - it only takes a second for a neck/spine to get broken.
If the small rat you're planning on introducing is very young, I'd be cautious when doing introductions. I've had very good luck in introducing 3 wk olds with adults in the past, but they were related. Also, they had been in constant contact since birth.
Also, when I've introduced a 5-7wk rat to an adult, the young one will try to get the adult to play with them constantly. Depending on the adult's personality, this can be okay or a terrible thing. Two of my adult males (one's getting neutered right now!) take any attempt to play by a young rat as an attack, and then leave large holes in the youngun.
So... be careful. Take your intros slow, and everything should be okay.
(Oh, and there've been reported problems with introducing dwarves in with regular rats - instinct kicks in, and the standard rats will often attack the dwarves.)