well one way of doing what you want to do is to copy your sound into sound edit. Using key layer you can silence the sound below the split point. Then use the sound edit in the new part.

You could change the notes below the split to silent notes in the sequencer note change after the recording, but this would probably be more long winded.

Experimenting with percussion is best done in the pads initially maybe? Step record some simple patterns as they loop, you can hear the results straight away, erase what you don't like etc. It's all the same (and more) in the composer.