Questions regarding DC-DC converters powering transducers
One thing to consider is how long the cable length is between each transducer and avoiding ground loops. If the system doesn't share a common ground (and the transducers are far away (meters) from each other, it might be good to buy isolating DC to DC converters (in this case you could use a single supply). 


  Is there a way to determine whether this current is enough per
  traducer?


Yes, if the manufacturer would provide the current rating for it's product. Either contact them or buy one and test the device to see how much current it uses (I'm willing to bet it's at least 24mA).


  Should such a filter be inserted in front of each DC-DC converter
  input or is it enough to use only one as in the drawing here?


It's probably not necessary to have a filter on the front end of the DC-DC converter (unless you think there is large amounts of noise in the area, in which case shielding the wire should be considered). What I'd be more concerned with is the pk-pk voltage on the output of the DC-DC converter. 

If using the current version of the transducer, for every volt of noise on the input of the transducer you'll get &lt; 0.003 mA of current error. So 10mV of ripple would be 30uA of noise.