Would like a pointer to a guide for modifying an existing FDB without having to rebuild from scratch. For example, adding one or more tables, perhaps linked with FK relationships to existing tables, adding fields to an existing table, increasing lengths of fields of existing tables, etc. What is allowed/disallowed?

If you have the rights you can do all of the above.

As already mentioned check out the documenation in particular search for "CREATE TABLE", "ALTER TABLE", "ALTER COLUMN".

You might also want to invest in the Firebird SQL book to get a better idea of what you can do with Firebird SQL.

http://www.firebirdsql.org/en/books/

Werner