For Hariyama's abilities: Go with Thick Fat for extra resistances against fire and ice, or Guts for extra assurance against status effects, whichever you think you're going to encounter more. Sheer Force removes beneficial secondary effects from moves but powers them up. I'd go with this one myself, but Hidden abilities are so hard to get.
For Gallade's abilities: Go with Steadfast if your Gallade flinches a lot, or Justified for help against dark types (note that's Justified is a Hidden Ability and therefore difficult to get).
Between the two: Gallade is a better choice for type coverage and is the only one of the two that can Mega Evolve. Gallade also has better stats. The only downside to it is that you'll need a Dawn Stone to evolve it. For its ability: I'd go with Justified because dark types are encountered a lot in ORAS. Don't worry about the nature in-game. It doesn't really matter unless you're playing online in a competitive game.