You can use my public plugins with your commercial, no-commercial and in-house projects without GPL-ish restrictions but you can't resell them with any means on any marketplace but you can merely share them as free with credits.
There is and will no support for older engine versions. All plugins will be updated after one month of new engine release. If you need to use our plugins with older engines,
- create new blank plugin with exactly same name,
- copy source folder.
- If there is no engine api change, it will work. If there is one, you have to fix them. Generally, Epic changes getters & setters and Visual Studio logs are enough to fix them.