I need to have PHP 7.1 installed, but the version from repo is 7.2
Is there anyway to downgrade it to 7.1 or just install 7.1?
As far as I know Solus does not allow us to add third-party repository, right?
Solus does no such thing...however, you'd need to find someone providing that package for Solus in a Solus repo. The reason there aren't really any, cause packages get into the main repo and managed there.