Will there be an option to mix "regular" views with compose, so existing UI open source libraries are still usable?

Yes, it’s possible even now, check samples

I will take a better look when I'll have my computer with me. But just to verify, I'm not talking about converting composables to views but turning views into composables

Compatibility works both ways

Good to know, it will help to a lot of people migrate

And thank god Compose will not be tied to platform releases and we won't have to wait a years to fix bugs)