First off, you have to understand that you can't just hire/contract with an F-1. They are only allowed to work under the OPT rules which means you have to work out their employment in the course of their educational course of study. You need to be working with the international students office at whatever school you're hiring these guys out of.
Second issue is that you can have people sign NDAs from now until the cows come home, but if they go back to their own country to exploit your IP it will be hard (and in some countries impossible) to pursue any recourse.
Third, you should be darned sure that any "contractors" you hire, are in fact, treated as contractors. Computer programmers/engineers have a disposition to be treated as employees by the feds.
Be sure to comply with the federal taxation law (notably W-8BEN forms) for any foreign entities you contract with. Further, if your "contractors" physically reside overseas, you better be aware that there are certain employee laws that you might be opening yourself up to.