It’s decently modified. To a person whose never used Blender before, they’d never see a difference. From a person who uses Blender, I can’t even tell the difference beyond some buttons being different. .blend files are compatible to be used within Goo Engine.
Yes. If it’s from the right version.
Goo nodes may not work in EEVEE, since it’s intended for GOO, but I wouldn’t be surprised if they did should you port them over.
They should. If they don’t, demand GOO for a refund. (don’t)
It’s free, and even it wasn’t, demanding a refund when something doesn’t work as you expect isn’t generally a productive way to incentivize developers on a small open-source project to listen to your feedback and implement your requests
Adding this, just for clarity: goo is a modification of Eevee, so is already working with Eevee.
However, if you open a goo shader scene in regular blender, the goo nodes are disabled as blender won’t know what they are.
Also, to “do goo shaders work with any material?” The goo nodes perform their own designed functions, so the question doesn’t quite work. Technically the answer is “sometimes”, but the nodes also have intended uses and may do something unexpected or undesirable if used outside their design intent.