That's... exactly the use case we were thinking about! We've been using Voice Memos to record song bits / mouth beats, and hopefully this is the better / smarter version of that.

Chops does support multiple instruments at a time -- right now, you can layer many drum and audio tracks (drum tracks play built in drum samples, while audio tracks are those that you record using your phone's microphone).

The app is MIDI-based, but it cannot trigger external software/hardware devices at this time. This is something that's on the roadmap!