gst for mess fee

Yes it is a composite supply, same GST rate as that of room rent.

If declared tariff is less than 1000, then it is exempt as per notification 12/2017-CT(R). Then no need to charge GST.

But please note that room rent and declared tariff are 2 different terms carrying different meaning. Declared tariff may me more than 1000 and actual rent may be greater than 1000 or even less than 1000.