When u air fireball you have to tiger knee the motion (d,d/f,f,u/f+p). this is so the fireball comes out at the beginning of the jump. This is so that initially the fireball hits them from the front, while you cross them up and combo. in other words the fireball hits them from the front and you end up on the other side.
I personally use the jab version of the fireball, but i think pokesYOU has a lil more akuma experience under his belt and i havent really experimented with the strong version. The fierce version normally is too fast and they recover before you can combo them.
the actual combo you should use when u land depends on if they block the fireball.
if they do block the fireball: land c.lp, c.hp, hp fireball.
if they dont…: land c.lp, c.hp, mk hurricane, hp dragon punch.
you need to wait for the fireball because if it hits, the fireball will cause them to stand up, allowing the hurricane to hit. If you simply do the hurricane while theyre still crouching, its NOT safe and ull get uppercutted or supered against anyone who knows what they’re doing.