well first of all, you can’t have just *a* rat. rats are extremely social and must live in same sex groups. single rats are depressed, less likely to bond with their people, more prone to illness (do to stress and depression), and live shorter lives. now that that is out of the way! 
something a lot of people don’t realize, in the wild, rats are also preditors. they frequently hunt birds, eat mice, lizards, and other small animals. if you have a dwarf breed rabbit, i would be concerned that the rat’s wild instincts could kick in. unlike dogs, you can’t teach a rat to control its prey drive. my sister was visiting once, and brought out my new little ferret, not realizing that several of my rats where playing in the living room. one of my big male rats attacked the ferret, and nearly had him for lunch. the only time i have ever been bitten by a rat was when i was trying to pull templeton off of vash. conversly, most rabbits are significantly larger than rats, and could seriously hurt one if it was mad enough. you can easily own both at the same time (i have), but i would suggest keeping them separate. better safe than sorry!