Barkingdog - Thank you for the website link, it is actually one I reviewed and
derived the idea of a 'safe room' from.
sugarcatmom - Thank you as well for the input.
The new cats name is Chompy, its a chocolate himalayan.
The aggression is expresses is primarily towards our younger resident cat, Hammi, whom would in the hierarchy of things be considered at the bottom.
Hammi is extremely curious and therefore regularly can be found waiting outside the door to Chompy's hideaway.
Chompy has developed a comfort for the second floor of our home, because this is where she has been left to explore most freely. Thusly at times upon immediately seeing Hammi when the door to the safe room is first open, will hiss, growl and sprint immediately after hammi. Both have claws so in other fights some swatting has been present.
What is difficult is when Chompy sprints full speed after Hammi upon opening the door. This I would like to prevent from occuring as I feel they could develop a bad relationship.
I have one of those plugins in Chompy's room (she was very upset when we first received her; now she is extremely loving to both myself and my partner when we are in the room). I could try them in more places in the house.
More vertical space is also an interesting suggestion. We have a cat condo in Chompy's room but the hammock idea could work. Or a shelving unit.
The 'open door' concept I think could work I just don't want them swatting through the door. But I will try.
Overall all good suggestions, thank you!