Be sure that any contract work you have done is covered by a contract agreement that includes a non-disclosure statement, and a statement that any inventions created under the contract are assigned to your company. Again, get advice from your lawyer.
And be sure that your contractors qualify as contractors. Companies have gotten in trouble by hiring as contractors people who otherwise looked a lot like employees (but didn't get benefits because they were contractors).
The Oregon Bureau of Labor and Industry has a page on their web site summarizing the difference between an employee and an independent contractor.