Welcome Mcm18!
I have never had close contact with either species, but someone told me red rumped parakeet are extremely territorial. That person compared them with the same level of territoriality of the pacific parrotlet (which I know well) and I would say that must be a good amount of aggressiveness.
Moreover, if the cage is small, I always think there is some risk or disease transmission as microorganisms that are not harmful for one species, might be to the other.