What STCW endorsement you will need to serve as Chief Mate will depend on the COI for the vessel. If it calls for a Chief Mate, you need to hold a management level STCW II/2 endorsement. If the COI only specifies Mate and you are the “chief mate” because you would be the deck officer next in rank to the master and upon whom the command of the vessel will fall in the event of incapacity of the master, then you would be OK with the operational level II/1 endorsement.
For Master, the tonnage of your STCW endorsement needs to be appropriate for the tonnage of the vessel, either 500 GT or More and Less Than 3,000 GT or 1,600 GRT/3,000 GT or More.
Not sure where you got 7 miles from. The requirement is operating beyond the boundary line, which may not be 7 miles, see 46 CFR 15.1101(b).