Your former employees may have grounds to file a claim against you if, for example, you laid off someone over the age 40 and two months later replace her with someone younger than 40; or if you replaced a disabled individual with someone two months later who has no disability. The same would be true for any protected class (race, color, religion, sex, age, disability, marital status, etc.)
If you laid off your bad performers and are trying to avoid rehiring them, you just have to be prepared for claims to follow. Laying off bad performers is tempting, but not the best way to manage such situations -- as you may be now discovering. There is no "magic" number of months after which you will find it safe to replace laid off workers with newbies. If you do this, you will have to take your chances that the laid off workers do not learn about it or that none of them are in protected classes or that none of them are litigious in nature.
If you want to fill jobs formerly staffed by laid off workers, the right thing to do is to contact laid off workers if their jobs are now available to see if they want to return to work. Even if you are offering less money for the same jobs -- which you have a right to do, all things being equal -- it would still be most honorable to give them "right of first refusal." Document your efforts, then if the former employees are not interested or available, go ahead and hire new workers.
Any good employment attorney can advise you on this matter. Researching it yourself may prove to be an arduous task, as a number of laws are involved. But if you want to do your own research, a good place to start is [url]www.DOL.gov[/url] which is the website for the Department of Labor.