income tax e filing registration

No problem you can give..Every communication bwt department and client it will come to your mail id, even income tax return acknoweldgement too..

NO problem will be there

yes, you can give your email id

email id not a prob, but u have to keep good track of all assesee , coz there would be all information related to all assesee routed to your email id, and you have to segregate all assesee wise, any mishandling may generate a mess.