Currently AD sync rule handles exchange. Can I have a sync rule just for exchange and remove the exchange sync rule flows from AD? In that case, should the exchange sync rule be set as dependency on AD sync rule?

As I understand, you want to have two separate Sync Rules - one for users (normal accounts) and the other one for creating mailboxes?
Yes, it is possible - just configure dependency to be sure that mailbox would be created for the account that exists.

Thanks Dominik. I am just wondering what will happen if I don't have the dependency rule.

It depends what those Rules would do. Without dependency for example you can have a situation where SyncRule for Exchange tries to export attributes for not existing object in AD

in this situation, with a dependency, you cannot use 'initial flow only', which is ideal for exchange attributes (you don't want the user to be moved back to the same db/server by FIM if an exchange admin wants to move them!)

with a non-dependent sync rule that only includes the ex attributes, what happens if it tries to run before the ad provisioning sync rule?  will it fail, and re-run after the ad rule has run and then succeed?

thanks ben. I got it solved. I didt set that db/server to initial flow only but i flowed the new data from ad to fim and pass it back to AD if there is any change.