I can really only answer for Soul Eater as i have not watched Negima yet, the Aria's (or mantra) used by the witches are themed on their powers and the animal/object they are themed around, with Blair, even though she's "a cat with a ridiculous amount of magic" her attacks are themed around Hallowween and pumpkins are a symbol of Halloween.

With Medusa, her theme is Snakes so when she says *"Nake snake cobra cobubra"* its a refernce to snakes (Nake and cobubra may be deliberate misspellings of Snake and cobra) while her Vector Magic seems to resemble snakes in how they move.

with the Mizune sisters, even when combined we don't see any Aria used their signature phrase "Chi Chi Chi" could be taken their aria as "Chi" is described as the sound of a mouse squeak on the [Wikia page][1]


  [1]: http://souleater.wikia.com/wiki/Mizune