Oracle Forms 6i on NT over a 10G database is definately not supported and has never been certified.
Forms 6i itself, as a product, was desupported on 1st January 2005.
Whilst there was some inter-operabilty between Forms 6i and AS/DB 9i you will need to lift your forms to 9/10 to work with a 10G database.
SQLplus has not changed greatly over the years and it’s no suprise that an 8i version would continue to work with 10G
as the plus bits are local and the SQL bits are mostly ANSI standard.
As far as compiling Forms is concerned, the compile process has a dependency on objects outside of the Forms Tool ( ie server objects ), and these objects will have radically changed from 8i to 10g – I belive that is the crux of your problem.