1. If I’m working on an app which has minSdkVersion=18, can I use an Adaptive Icon for app’s launcher icon by defining the additional res folder mipmap-anydpi-v26/?
Yes, that’s exactly what the blog post said.
2. I still need to keep the PNG files for older android versions right?
3. Can I generate the Adaptive Icon from an SVG/PSD file or I have to define it directly in XML format in Android Studio?
Android devices can’t read SVG or PSD files, so you have to convert them. Android Studio allows you to import vector images or you can use third party tools such as Shape Shifter (which also has an import function).