I cant say what is the preferred method for our local rescue - but they recently had a new "cattery" fitted.
It is primarily individual runs, of which are large enough to house those who come in as pairs etc. However each side panel of the single runs can be opened out into each other should the cat next door want to pop in. They used this for a few sociable cats that just wanted company, but were happy going back to their own pen. This is good because you have the choice and those who prefer to be only cats still have that privalge
They converted a room into 2 playgroups. Much much larger and mainly used for litters of playfuls kittens & their mum (if the kittens still need her). Decked out in cat trees, toys etc.
They also have the "hospital" bit for isolation.
Anyway! Good luck, i'm so excited for you!! Cant wait to see pics of the end result, whichever method you choose