Maybe I can help you on a few of those points. First off, feel free to fork anything you feel like, whenever you feel like something needs to change. Also feel free to push any changes you make back to the original project. The project owner can decide what to merge in. As for lisening, please note Im NOT A LAWYER, however, I think I can offer some general knowledge. If code is under a typical open source license, the code at that particular point in time is free to be viewed by anyone. A lot of open source licenses, though not all, are considered 'copy-left', in that any code DERIVED from the liscensed code also has to follow the same license (GPL comes to mind). This helps from people 'stealing' your code at a later point under a closed license. As for taking code snippets, it depends on the license of the code. If you grab a piece of code from a GPL codebase, whatever you're using that code in also has to be GPL licensed. You are in essence taking their code, and modified it. Learning more about different open source licenses might be something to look into, depending on how far you want to delve into this
As for making people pay on Play store for open source projects, its certainly acceptable. A lot of people make money from open source software by prividing some form of 'support'. Whether thats tech support, or whether its compiling, packaging, and making it available (as you might do for an android app) you are providing some sort of service aside from the actual code that people will pay for. I've seen a few projects go this route. Hope this gives you more answers than it does questions! The OSS community I've found to be a pretty easy going group of people, so long as you follow a little common courtesy, and have a bit of common sense. Cheers!