wants to migrate toHeadstart 6i contains packages, libraries, forms, menu etc. So theoretically after recompiling the headstart environment it could be used with the higher version of Oracle Forms and the database. Do you know if the headstart environment (library/packages) has been changed? If the headstart environment isn't changed I would recommend to upgrade you headstart environment since there where important bugfixes.
Development Tool -> Developer Suite 10g with headstart 6i (same version which currently in use)
Database -> Oracle 10g
and can I use application server other than Oracle.Don't know if you can use a different application server. I have never seen such a combination.