As Koz has said, there may be a bit of "cross talk" (the signal from one microphone circuit bleeding through to the other circuits), but for your application that doesn't matter since you intend to mix them all together anyway.
It is even possible to run all of the microphones from the same batteries, but there is an important problem that needs to be overcome if you want to do that. The problem is that the microphones will interfere with each other due to signals passing through the battery.
Going back to the first diagram
If each microphone has its own 2k2 resistor, that will provide partial decoupling between the microphones. That resistor value can be increased and so improve the decoupling, but doing so will also drop the voltage across the microphone capsule. If the voltage drops too low the capsule will not be powered sufficiently and will distort, or stop working altogether. If you want to experiment a bit you could try adjusting the value of the resistors (too low a value could cause an unacceptably high voltage on the microphone and destroy it) and you may be able to find suitable values to provide adequate performance.
The very much easier - and better - method (though more expensive on batteries) is to use a separate circuit for each microphone.
There should be no problem putting them all in the same box, but they should all be "earthed" to the box at the same point (a nut, bolt and a couple of washers bolted through a hole drilled in the box should do the trick).
