If Sonata is mostly just funny teenager, Aria is grumpy one. Aria is just more annoyed about things (it's not much different than Rarity's or Rainbow Dash character). She will be harder to redeem - yes. But that just will make things more interesting!
Adagio, however, the one who are true villain. Smart, power hungry, ambitious. If it's not impossible, it will be really, really hard to redeem her. But that will make things even more interesting, right?
Oh yeah, Discord didn't understand friendship but his abilities are useful so he should be redeemed yes :\