Certain Cases support it. My CM Storm Sniper supports ATX and smaller. But most don't. I couldn't find any definitive information on whether the HAF 912 does though.
Yes, as long as you aren't using an older model mini ITX board. The mITX's 4 holes match the pattern in the ATX case as well as the I/O cutout.
http://en.wikipedia.org/wiki/Mini-ITX