I would absolutely recommend not to reuse the database schema used by other applications together with Mendix. The only exception I could think of consist of read-access to the Mendix database.
Mendix uses its own structure and id-generation, in a generic way such it can be applied to all database types. Future releases could theoretically change the setup, which may break your application.
For shared data, I would recommend to use either APIs or the database connector to send (custom) queries to a database; but next to that: let Mendix run in its own space.