0-Azula-0
Katara should be with Aang. Im am so tired of seeing people pair her with Zuko. For one, Zuko LOVES MAI!!! She got herself arrested and throwm in jail for the man to save his life! If that isnt love, I dont know what is. And for two, Aang is madly in love with the girl. He even said himself the only reason he was really doing this saving the world thing is to be with Katara :3 which I think is adorable. Aang and Toph go well together I must admit, but really... is he gonna handle all of her badassness?
You're absalutely right! Zuko and Mai are the only ones who can really understand eachother. I can't imagine anyone else understanding Mai... she's just so... different? (she's one of my favorite characters) And Zuko has had so much pain in his life, which Mai can understand having coming from a royal/noble family where she was heald to rigid standards like Zuko. They knew eachother since childhood, where Azula (even then) picked on the two as a "couple" of sorts. Zuko and Mai make sense, and I like it.
As for Aang being to young for Katara... how? It's only two years (when you ignore Aang being stuck in the ice for 100 years). Zuko is two years older than Katara, so it's just the same age difference.... Aang obviosly liked Katara from the start, and though she was confused even in "Emerald Island Players" and rejected him more or less, Katara realizes she likes Aang in the end. They're both young and playful at times, but both can be serious and strong willed. They're perfect together...
As for Toph, I don't know... I can see her hanging out with Uncle Iroh and playing Pai Sho XD but I can't really see her in a relationship with anyone.