1. Mini Itx(6"x6") and microATX(9.6" x 9.6") differ greatly in size. FlexATX, like my shuttle FN41(7.5"x9.5") mobo is a bit smaller. Also a lot of microATX boards dont have onboard graphics. They all normally use an ATX power connection so the opus works with either.
2. It'll work. The larger PSUs in desktop sets are for add ons like multiple optical drives and sound/graphics cards.