Mega Salamence doesn't have much of a reason to use Outrage outside of Wash Rotom. Return/Double-Edge is its new nuking attack; consider Fire Blast over Outrage to hit specially defensive Steels. Regular Salamence pales in comparison to Mega Salamence because the former struggles big-time with Fairies without the inaccurate Iron Tail. Mega Salamence, on the other hand, can simply hit Fairies hard with Aerilate Return/Double-Edge.
Tyrantrum is a better Head Smash abuser because of its higher Attack (and access to Dragon Dance to boost its Attack along with its Speed). Rhyperior is also worth considering for its better physical bulk and Solid Rock to cushion super effective hits:
Tyrantrum:
-Dragon Dance/Rock Polish
-Head Smash
-Outrage
-Earthquake/Superpower/Crunch
Nature: Jolly/Adamant
EVs: 252 Atk/4 Def/252 Spe
Item: Life Orb/Lum Berry
Ability: Rock Head
or
-Head Smash
-Outrage
-Earthquake/Superpower
-Crunch
Nature: Adamant/Jolly
EVs: 252 Atk/4 Def/252 Spe
Item: Choice Band
Ability: Rock Head
Rhyperior:
-Stealth Rock
-Earthquake
-Stone Edge/Rock Blast
-Ice Punch/Toxic
Nature: Adamant
EVs: 212 HP/44 Atk/252 Def
Item: Leftovers/Sitrus Berry/Rocky Helmet
Ability: Solid Rock
Mega Diancie is better off as an attacker, rather than a supporter. Also, a Quiet nature is a waste of Mega Diancie's good Speed, so Diancie's regular form takes precedence over its Mega in that case. Give it Mental Herb so it won't get crippled by Taunt once.