replacing the traditional instrument with a second AI
The second AI would not meet the intent of the design requirement 23.1321:
The instrument that most effectively indicates the attitude must be on the panel in the top centre position; [/font]
in the position of the TC.
if you disconnect a pitot/static instrument from the pitot/static tubes, do these connectors auto-seal or do you have to put some kind of plug in them, to prevent false readings on other pitot/static instruments?
Whoa there! The discussion of the pilot disabling the turn co-ordinator is definately limited to that instrument only. Any maintenance on any pitot static or vaccum instrument very certainly requires properly trained personel. In particular, you should expect that airspeed indicators and altimeters are required to be servicable for any flight.
That said, an instrument which is legally not required, can be temporarily labled "inoperative" in place if secured properly. This is with the expectation that it will be removed by maintenance personel in the near future, either permanently, or for repair. The only reasonable exception I have seen to this is where a secondary instrument like an ammeter is in a cluster (can't be singularly removed) and it has been disconnected, and replaced with a new, separate ammeter. Then you'll sometime see a permanent "inoperative" on its face.
The replies on this thread are not intended to confer maintenance authority to pilots. What you have here, is guidance telling you what, and where to look for the regulatory information you need to properly care for the aircraft you are flying. Know your privilages, responsibilities, and capabilities! Saying that you read it on PPRuNe is going to look pretty poor, when challenged by your local authority for unauthorized maintenace of an aircraft!