You would typically put customizations in either the :config or :custom sections of the use-package declaration for the symex package, unless it is specific to an optional package like symex-ide or symex-evil, in which case you should include the config with these packages.
You should avoid placing customizations with symex-core (put them with symex, instead) as, if Symex were to be hosted on a package archive, symex-core would not need to be explicitly declared in your config at all, and would be installed behind-the-scenes as a dependency.