Hi Josephin!!! I have now two new cats and my older ones are not used to them. How can do? I'm so happy to see Thea is feeling better.
The best thing is to tale things slow, let them be apart and if possible have a svreendoor between them so that they can get to know each other without any fighting or dominance trouble. It might tale some time for them all to get along, just so you are prepared for that. If your other cat is very old, this might be a bad idea and please look for signs of depression. Good luck <3