I don't have the skill points in it to confirm 100% for you but generally it should. It's just major and major or minor and minor buffs that won't stack. Igneous is a unique buff without a specific name type and momentum provides Major Brutality.
Yes the two skills stack. Momentum is major brutality and igneous weapons is a heavy attack buff. The two definitely stack, and you get the 5% minor brutality buff from using an earthen heart ability while gaining ultimate. You must be gaining ultimate though to get the minor buff (you don't get the buff if you are holding an ultimate)