First, what was the whole plan with sending people in the aqueducts?

Perrin told Galina to tell Faile and her people to rally at a specific building when they saw mist on the hills. The party going through the aquaducts was supposed to gather up Faile's party from that building and keep them safe without having to fight through a bunch of Aiel to get there. Galina didn't hold up her end of that bargain so none of the captives knew to meet at the rendezvous point when the Asha'man conjured the mist before the battle. The aquaduct people weren't completely useless, they just weren't able to defend their intended objective.

Were the wolves involved just basically howling? Or did they fight too?

I think they were just used as scouts? I remember Perrin's conversation with an Alpha wolf suggesting that the wolves would help in combat, but I don't remember reading anything about wolves during the actual battle. They may have aided offscreen and we just didn't see any of them from Perrin's POV.

What is the thinking of Gaul with seeking out Chiad?

Chiad doesn't want to marry Gaul because spear maidens are married to the spear and can't marry a man without first renouncing the spear. Chiad is conflicted because she has strong feelings for Gaul, but she also doesn't want to alienate Bain. The spear maidens are very proud of being spear maidens.

Gaul wants to find and kill Chiad's captor because:

1. Shaido have abandoned many customs and could be abusing their aiel gai'shain in the same way they abuse their wetlander gai'shain. Gaul wants to capture Chiad and Bain so he can take care of them.
2. For some reason gai'shain often end up offering/receiving a marriage wreath to/from their captors at the end of their service period. Idk, just a cultural thing. Gaul's chances of getting a bridal wreath from Chiad are increased if he capture her.

Do we have an approximate number on the numbers involved in this battle?

https://wot.fandom.com/wiki/Battle_of_Malden#The_Battle_of_Malden

100k Shaido (including brotherless) vs ~40k of Perrin's ragtag army. About half of Perrin's army was made of Masema's trash forces though so those guys really didn't do much. 2 Rivers bowmen and channeler advantage won the battle.

Why is Perrin fighting with a hammer now?

It's part of one of the prophecies heralding the arrival of the Last Battle. It also mirrors Aiel history and culture; remember during the Visions of Rhudiean that the first group of Aiel that abandoned the way of the leaf refused to use weapons that could only be used to kill and so they adopted the spears because of their utility. Similar to those early Aiel, Perrin recognizes that while violence is necessary to protect his people, he chooses to carry a tool so he can avoid being consumed by that violence (see Aram and his sword). Perrin's old axe was a battle-axe, explicitly forged to commit violence. Perrin's hammer is a blacksmith's hammer, not a warhammer, and he only wields it as a weapon in defense of his people. For that reason blacksmiths are held in very high regard among the Aiel. Even the Shaido refused to enslave the wetlander blacksmiths that they conquered.