Is this formula for calulating the energy requirements of pets applicable to all animals?
No, I would not use this formula for rodents. The "3/4-law" is usually used only for mammals with a mass of m>10 kg and was derived by Kleiber from limited empirical data sets. 
For mammals with a bodymass below 10 kg the prefered exponent seems to be 2/3 which was derived from the relationship of volume and surface of the body and heat energy as the main source of metabolic rate. See here.

Either way this are only approximate calculations I would not trust too much as exceptions occure frequently and also the age and/or physical state can make a difference.