How do i fix it if i forgot to uncheck it for some "not that main" application for particulat type. Say, i have firefox, mozilla and mousepad for XML files, by default i open XML files in mousepad and i want Mozilla to disappear from the popup menu and from Recommended applications - how do i do that?

You need to remove "application/xml;" from the "MimeType" section of /usr/share/applications/firefox.desktop

How do i make applications appean in Thunar's Recommended applications and Other applications sections?

Particullary i want to make ghex2 available for all types of files in Other applications section.

Another question - is there a way to make "Use as default for this king of file" unchecked by default?

How do i fix it if i forgot to uncheck it for some "not that main" application for particulat type. Say, i have firefox, mozilla and mousepad for XML files, by default i open XML files in mousepad and i want Mozilla to disappear from the popup menu and from Recommended applications - how do i do that?