Database design for a table with both bulk insert and single row insert
1a) If the two insert methods create the same data then making them one table seems to make the most sense. After a bulk load I would recomend running maintenance scripts to make sure that all indexing is up to date.

1b) The only way to know if the bulk insert and the single insert would effect each other in this specific case would be to test it. With sufficient hardware this would not be an issue.

2) Both SQL Server and Oracle are mature products that can handle what you are describing very well. It sounds like you have a preference for Oracle which should be able to handle what you are attempting.

3) SSIS would be a great solution for what you are describing concerning loading millions of rows into SQL Server.