Would an atx 12v be compatible with a micro atx motherboard

The form factor of the motherboard doesn't really matter for this. As long as your motherboard accepts the voltages supplied by the PSU, you're good. Just double check the specs before you power it on.

nope fire it up, the board will draw only what the parts need

atx power supplies usually support both micros 4 pin cpu layout and normal atxs 8 pin by having the cpu power cord a 4+4 cable.