Look at the creaturepackzombie xml for how to setup the xpath. You need to target the zombie AND the probabilities. For example:
<insertAfter xpath="/entitygroups/entitygroup[ starts-with(@name, 'zombieHospitalGroupGS')]/entity[@name='zombieBoeFeral' and @prob='0.1']">
<entity name="zombieGuppyDoctor01Feral" prob=".01" />
<entity name="zombieGuppyDoctor02Feral" prob=".01"/>
<entity name="zombieGuppyDoctor03Feral" prob=".01"/>
<entity name="zombieGuppyDoctor04BlackFeral" prob=".01"/>
<entity name="zombieGuppyDoctorFemale01Feral" prob=".01" />
<entity name="zombieGuppyDoctorFemale02Feral" prob=".01" />
</insertAfter>
<insertAfter xpath="/entitygroups/entitygroup[ starts-with(@name, 'zombieHospitalGroupGS')]/entity[@name='zombieBoeFeral' and @prob='0.2']">
<entity name="zombieGuppyDoctor01Feral" prob=".02" />
<entity name="zombieGuppyDoctor02Feral" prob=".02"/>
<entity name="zombieGuppyDoctor03Feral" prob=".02"/>
<entity name="zombieGuppyDoctor04BlackFeral" prob=".02"/>
<entity name="zombieGuppyDoctorFemale01Feral" prob=".02" />
<entity name="zombieGuppyDoctorFemale02Feral" prob=".02" />
</insertAfter>
Just 2 sample entries of many, but you can see that in the hospitalgroup, it looks for a zombie named zombieBoeFeral, and a specific probability, and if found it appends one or more new zombies with a specific spawn probability.