first of all, very unusual to say the least.
Second of all, youre a VERY VERY brave person.
But, I have to say, well done, and if it wasnt for you, the cat may not have lived. You have admitted he needs to go to a new home, and are in plans of doing this.
1 problem solved.
I did see goldfish, rudd and koi, but you are working on a new home for them too
2 problems solved.
with regards to the black phantom tetra, imo, if theyre happy now, leave them. fish ADJUST. if it wasn't going to work now, then you would have known and theyre still gooing strong. I would NOT move them as its unfair (just dont put them in the pond that could be TOO cold lol). They may be surviving so well as it isnt really a cold water tank, its technically a "room temperature tank"
3 problems solved
with regards to the new tank, i LOVE the layout, very cool, would be amazing if it stays that way
, but overall i like it.
What kind of fish do you like? tetras / livebearers / cichlids?
EDIT: I see you already covered most of my points (seem a little useless now lol)