mini-ATX and mini-ITX are interchangeable as far as mounting and connections. The difference is that ITX only supports low power VIA chipsets that only need passive cooling.
A switched-mode PSU will definitely make less noise in a low power situation. Usually, the fan is the source of most noise. You may discern a slight hum, and that depends on the quality of the PSU. If you pay enough money you should get a near-silent PSU. Other factors may make the PSU in question rather noisy, and they are related to design and manufacture.
FlexATX is an Intel specced design that is usually smaller (to fit smaller cases) than a standard-sized PSU. It is compatible with ATX, mini-ATX, and mini-ITX motherboards.