It's not clear to me that Rocco commanded a regime in Nevada. He may have been in charge of security for the compound (as we saw after the shooting), but that's not the same as a regime. Michael didn't need regimes in Nevada as Vito did in NY.
Michael sent Rocco on the mission so Roth would be dead. He was reasonably certain that Rocco would kill Roth because neither Roth nor the Feds were expecting an assassination. If Rocco survived, swell. If not, Michael didn't need him anymore.
TB, this may all be a matter of semantics. While regimes were not necessary in Nevada for combat since it was an open state, "Rocco and his men" certainly sounds like a regime. Sending Rocco to murder Roth may or may not be what a don would really do, but in the film it was probably for dramatic effect since we knew who Rocco was.
