Use the M7 bayonet. The M9 will work, but I am told you may have to do a slight bit of grinding on the bayonet lug for it to fit right(I believe that both M7 and M9 were issued for the M16).
The 590 uses the standard issue bayonet for the M16, which is currently the M9. The M9 is a POS, and an expensive one at that. The older issue M7 will work as well, and is inexpensive.