They will manage... Orcs generally have quite high initiative... only warmongers are slow. But my paokaies immediately fly to the castle, they are attacked, the leader attacks the centaur, it moves, kills someone... the paokaies eat and then fly back)))

This tactic is useless because paokaies themselves have low initiative... they won't reach the enemy in the castle on the first turn, they might attack on the second, eat corpses on the third, and only fly out of the castle on the 4th turn. Most enemy monsters will move 5 times, and 80% of light/dark elf and Inferno units will move 6-7 times.
Moreover, even if a cyclops reaches and breaks down the gate on the first try, it will need to retreat on the second turn; otherwise, any weak defending unit will simply block the cyclops at the gate, preventing paladins/leaders, warmongers, shamans, and goblins from passing.
Centaurs will not shoot very effectively from behind the walls, and it's not certain that they will shoot first, and of course, the defending archers or hero will primarily take care of the centaurs + towers!
By the way, killing a stack of the defending army for the paokaies to feed on is not that easy, because only on a map where there are hiding places for the 2nd-tier unit + Haghesh, etc., can you gather a decent number of centaurs, but it's not certain that your opponent won't also have "thick" stacks, for example, red archers.
Only the Inferno faction might have a problem with eliminating centaurs in the first round, but everyone will be able to summon reinforcements, except for the pit lords and fire demons - and in the second round, the centaurs will already be blocked or shot down.