IMHO realistically for most people the ratio between "your own ideas" and everything else should be like 5% or so. If you're exceptionally gifted maybe up to 20%.. (unless you're writing fiction)
If you mash together two ideas, is the new composite idea yours?
I'd say it's yours. In that frame, there are lots of ideas.
Lets assume there are 10 000 known ideas. Then there's 10^8 combinations of two ideas, and 10^12 combinations of three ideas. That's a lot of ideas, even for the internet! I bet not all of them are named. And different people are going to frame ideas differently.
I also believe trying to form your ideas in reference to existing knowledge is a great way to learn existing knowledge.
