I have built computers on noth those cases at school (although Haswell)
http://www.tomshardware.com/forum/id-2625445/build-log-consolidated-middle-school-builds.html
If you are using a locked CPU you can use H170 motherboards. What CPU are you thinking of?
Skylake runs out of voltage before it runs out of thermal margin, most of the timer, so the CPU cooler can be more modest if you overclock.
Do you have a budget?
The most feature-full and robust mITX is the ASUS Maximus VIII Impact, and it is also very expensive. For an overclocking system
ASRock Fatal1ty Z170 Gaming-ITX/ac
Asus Z170I PRO GAMING
would be my top choices.
ASRock H170M-ITX/ac
Gigabyte GA-H170N-WIFI
with a locked chip.
For mATX there is much more choice
The...