For the Chopin etude: You should not be scared of 'fast' pieces if you want to play etudes ..and that too, the CHOPIN Etudes ! That is the point of learning them ...to solve your problems. In fact, I would recommend you to pick some aspect of your technique which is not good and you would like to improve and then pick the etude which addresses that aspect.If speed is your problem: Op. 10 no. 4 is a good choice. Op. 10 no. 8 too (mostly for the right hand though.) Op. 10 no. 12 is one of the good etudes to begin with. I wouldn't advise you to go for the thirds or the sixths or the chromatic etude yet.