Alfa Steve - wow, 2 child seats in a Brera, that's got to be a record!! Had to wave goodbye to mine recently due to tiny feet.
My experience (researched extensively in a doomed attempt to keep the car):- Isofix mountings are there for show only and no seats that we found came close to fitting the car. First stage seats were out (rear facing) as the gap between front and rear seats is too tight. As a result my car barely moved in the first year after madam arrived so good for keeping the mileage down!
Stage 2 seats (forward facing) do fit but choose carefully and test first. We ended up with a maxi cosi Tobi (around £170 I think non-isofix) which fit really well. Not the end of the story though as the front seat had to be so far forward that it made for quite a cramped ride for the boss. Eventually gave in, kept the seat and swapped the car. 159 TI great fun by the way thoroughly recommended!
Don't know about stage 3 I'm afraid.
One caution note - Alfa advertise a seat in the accessories catalogue - I've seen it and personally would not dream of strapping my little princess into that contraption (my opinion only of course).