OK, so some of your questions are not really relevant so I’ll try to help out on some of the underlying concepts.
I2C is a bus. So the pins exposed on each socket are the same physical pins… plugging a device into one socket should be no different to plugging it into any other socket. (and to verify the pinout, you can look at the Spider schematic PDF and see that they are all using IO11 and IO12)
In a Gadgeteer sense, Gadgeteer extends the logical (software) I2C bus design away from what the core netmf definition is, so in the ctor of the GTI I2C bus they use the socket - for example, the Seeed Gyro module is an I socket device, check out this source code:
And you can see it’s ctor looks like:
Socket socket = Socket.GetSocket(socketNumber, true, this, null);
// Init I2C
i2c = new GTI.I2CBus(socket, 0x68, 100, this);
So if you’re using the GTI interface (which you can see in http://gadgeteer.codeplex.com/SourceControl/latest#Main/GadgeteerCore/Gadgeteer42/Interfaces/I2CBus.cs ) you can use the socket differently, otherwise you can use the non-Gadgeteer approach like the I2C example that Andre pointed you to.
hope that helps