List of all skill that work with speed mix

Knight
Sword Blast
Double Crash
Brandish
Piercing
Grand Cross
Sword of Justice

Mech
Mechanic Bomb
Spark
Grand Smash
Impulsion

Pikeman
Critical Hit
Tornado
Expansion
Chain Lancer

Fighter
Impact
Triple Impact
Brutal Swing
Rage of Zecram
Avenging Crash
Destroyer
Cyclone Strike

Archer
Wind Arrow
Perfect Aim
Avalanche
Elemental Shot
Bum Shot

Atalanta
Farina
Bigger Spear
Twist Javelin
Fire Javelin
Split Javelin
Lightning Javelin
Storm Javelin
Extreme Rage (not 100% sure because of the huge delay it's hard to see)

Priestess
Multi Spark (not 100% sure I think there is only a small difference)
Vigor Balls
Resurrection
Extinction
Glacial Spike
Chain Lightning

Magician
Flame Wave