Thank you for your feedback! Unfortunately, clipboard sharing between other applications is one of Pixelorama's limitations, and we're not sure how to deal with it yet, but we are aware of it and we will try to do something about it in the future.
As for your other suggestion, version 0.9 will come with an improved selection system, which improve the rectangular selection tool a lot, possibly add new selection tools, and add all the other common features that you can find in other software!