Is this suitable?

A lot depends on how happy you would feel seeing hens in a small space.  I bought a coop and run said to be big enough for 6-8 hens but despite only having three they looked like caged birds to me.  The size of housing you describe is technically big enough for a couple of hens even if you cant let the hens free range, but will you be happy seeing them in a small space?  Could you add a run to the aviary?
If you decide to go ahead they will need a nesting box and you need to provide roosting bars in an area that you can enclose (hens suffer badly in draughts)  You can cover the floor with bark/ hemcore.
You will also need to think about preventing boredom if they cant be let out
Good luck with whatever you decide

Mine free range when I am home and have never attempted to fly anywhere, only onto picnic tables and chairs. Obviously some do, u could get a breed that isn't prone to flying

The trouble is the cats aren't mine so I can't guarantee it's the same cats so they get used to them and I don't know their personalities.

The neighbours on one side have dogs so hopping the fence would be very risky for the chicken even if it's just occasional.

We do have a bit of lawn at the other end of the garden so could perhaps pop a temporary pen there for a bit of scratching about but it wouldn't be that big. Our gardens not giant and quite sloped so it's terraced into three (patio - with the aviary, pond, lawn).

I hadn't thought of quail! Can you point me at a site with some good info. Can you just use the eggs like you would chickens - but a lot more of them I guess?

Dominic, which is that you have - normal chickens, bantams or quail?

It's not building it that's a problem, it's having space to put it We're in the middle of a 70's housing estate so the gardens are only the width of the house and not that long. Extending the current aviary would mean covering part of the house blocking off a window/door to the garden and on the other side of it the steps to the rest of the garden. We could maybe set up a second run on the grass at the end but there is no way to link the two as the pond is between and it's 5' difference in ground level.

Will they not sort of hop up the trees/shrubs and then over the fence? The slope means you can hop off one level and land half way up bush on the next.

hi sassy

Personally I wouldn't keep them together but there maybe someone on here who does

Their feed requirements are different and Katie Thears book says quail are not winter hardy, however, there is conflicting advice about this on the internet. I don't know about how they would get on together but as I said personally I wouldn't keep them together.