Is it permissible to marry the granddaughter of the maternal aunt, knowing that I was breastfed once by the maternal aunt, and is it possible to follow a different madhhab than that of Imam Malik on this issue?

The Maliki and Hanafi schools hold that a small amount of suckling and a large amount of it establish prohibition [of marriage], whereas the Shafi'i and Hanbali schools hold that less than five sucklings within the two years of infancy do not establish prohibition. The latter opinion is the preponderant one. It is permissible to switch from one madhhab to another if the ruling of Allah and His Messenger becomes clear regarding an issue, or for its permissibility, or under conditions including: not combining between the two madhhabs in a way that contradicts consensus, and not following dispensations [only]. Based on this, there is no objection for the questioner to follow the Shafi'i and Hanbali madhhabs regarding the issue of suckling. He is permitted to marry the girl from whom he suckled once from her grandmother, but piety and considering the other opinion are advised.