A question about the use of static reference resistor and balancing a floating source for an instrumentation amplifier
1) If I use the diagram in my LTspice sim above as a setup for many
  floating inputs, should I use a 10k to 10Meg resistor for Rearth? In
  sim Rearth gets rid of or reduces the interference through the earth.
  But in practice how does it actually do this and is that really
  needed? And what if I don't earth AIGND is that still needed?


You're simulating the parasitics to ground, so whatever that is in your system, that's what the resistance to earth ground needs to be. Rearth could represent and actual material with an impedance of around 10MegΩ or it could be an approximation of air, it depends on how you think your grounds will be tied together. A hand held unit would have a configuration as shown above.

There could be a different configuration by moving Rearth between V_interferance_1 and C_leakage_1 and use only one ground. Part of it's purpose is to limit the current from the circuit, however this would depend on the setup of the physical system. 

If there were a very high gain on U1 I might also add a resistance for air on C1, or if your sensor is high impedance. The resistance of air would be ~10^8 ohms (for worst case) and check then current and see if it affects your signal.


  2)I have 5 type of DC outputting transducers. By STP I will guarantee
  the imaginary impedance balance but there might be source resistance
  unbalanced. Should I measure their source impedance seen by one and
  add the same source resistance to the source ground to make R1=R2
  here? In other words should I make R1=R2 even though R1 source
  resistance is &lt;100 Ohm?


If you plan on matching with source resistors (if you need high bandwidth and transmission line matching) to the source then these would represent actual resistors on the source end. It could also include the resistance of the cable. These resistances will form and RC circuit with the cable and the amplifier and limit your bandwidth of your circuit. 

Run an AC analysis and check the bandwidth, if the cutoff is too low, then make changes to your circuit (that you can realize in the physical world) and simulate again.


  3) Would adding a series resistor to the source ground to balance the
  lines have any effect on cross talk during multiplexing?


The series resistance to ground shown in the diagram below is for referencing the sensor to ground. Because the inst amp has a very high input impedance, if the sensor is floating, you can have common mode range problems and rail out the signal, so you ground out one side of your sensor and reference it to ground with a high impedance resistor. 

This shouldn't affect multiplexing in the circuit shown above, but you have not mentioned anywhere in the OP where a multiplexer would be placed. It would depend on your sensor, and if it would tolerate the noise from multiplexing in front of the amplifier. If multiple amps and a multiplexer after the amps is used, it would not affect the signals before the amps.