Stack Flag

Yes, All "A" Skills stack with another "A" skill that comes from weapons, seals or some certain c skills (namely Infantry rush)

A good thing to keep in mind is the difference between in-combat buffs and buffs that take effect outside of combat.
Buffs that visibly show up outside of combat on your units stat line do not stack, only the highest applies.
While buffs that take effect upon initiation of combat stack infinitely unless mentioned otherwise.