The correct option is C Too
Here, 'too' is correct because it is usually placed at the end of a clause and impliesalso or as well. Here, it implies that we, and also they, live in a small flat.
'Either' is incorrect because it implies one or the other of two; it does not matter which.
'Neither' is incorrect because it implies not one nor the other of two things or people.
'So' is incorrect because it is used to show the size, amount or number of something.