Oracle SQL Developer supports mysql and third party database in a migration context in that it can be used to migrate third party databases to Oracle. It is not designed to support those databases as an IDE. As of SQL Developer 18.2, it doesn't look like it supports migration of Informix to Oracle.
SQL Developer 18.2 Documentation:
...and here is a link to Migrate to Oracle Database with SQL Developer:
Oracle Migration Workbench does support migration from Informix to Oracle:
We don't allow informix connections in SQL Developer - what you're trying will never work.
Very good. Thank you both for your time!