hello , can we still use the legacy revenue management when ARM is enabled?

Nope. And there's no undo on enabling ARM either so I'd be really careful with it. I believe the only thing that will technically still be legacy is an "action" you take against an order that was existing when you flipped the switch will still use legacy. So if you had an existing SO that was put in before you turned on ARM, it will use Revenue Commitments and Revenue Schedules instead of Arrangements and Plans. Anything new goes on ARM though.